If you've been getting buzz from your friends list about Dreamwidth, you've probably been hearing about OpenID in passing, even if you've never heard of it before aside from that little confusing option when leaving a comment. Basically, OpenID is a way to comment at other sites under your own name, without having to go through absolutely all the hoops that site requires, without giving them your password. Even if you're never planning to get a Dreamwidth account, you can still comment to your friends' journals over there, so long as you have an LJ account. You can even make a friends page, if you want to collect all their posts in one place, especially to read any locked entries. (On Dreamwidth, it's called a reading page.)
If you're a paid LJ user, you can syndicate the public bits of your friend's account from Dreamwidth to LJ. If you're not a paid user, you can still use the 'Add Feed by URL' box at the bottom as a search box to see if someone has created a syndicated account for them. Their account's feed URL is going to be http://exampleusername.dreamwidth.org/data/rss (replace the exampleusername bit, of course).
If you're thinking about getting a Dreamwidth account later, you can still set up your OpenID account now. There will be a feature to link an OpenID account to a regular Dreamwidth account later on. There's also no need to hold off on getting an account for fear of screwing up the plug-OpenID-into-regular-account thing. The feature is planned to work by taking an existing regular Dreamwidth account, and validating to an OpenID account while logged into that account. Details about how to actually do it will be forthcoming in the FAQ once the feature actually exists.
This is where you sign in. (You'll also see a little "Log in with OpenID" link next to the login form.) After you've signed in, you will be prompted to set and validate an email address. Do that. Set it here. You will be sent an email to confirm. There are two major reasons why you should set and confirm an email address. First, if you set an address, you can get emailed replies to your comments there. Second, OpenID users without confirmed email addresses are treated as anonymous users as far as commenting goes. OpenID users with confirmed email addresses are treated like regular users for comments. (This goes for LJ too, by the way.) [Edit: Third reason: OpenID accounts with set and confirmed email addresses will get invite codes once Open Beta hits.]
OpenID users are assigned an arbitrary placeholder username. This takes the form of ext_ and some number (they're assigned in order). Your Dreamwidth OpenID profile and reading page are going to be viewable via that username (but not a journal, since OpenID accounts don't get one of those). Because the internet is crazy, the underscore will be replaced with a hyphen. For example, I was the lucky 4th person to sign in with OpenID; thus, you can see my LJ's OpenID reading page and profile here:
Notice how the journal page totally fails to show up, though: http://ext-4.dreamwidth.org/ (this is normal, at least for the moment).
Do not panic if you go over to Dreamwidth and see that an OpenID account under your name was created even before you decided to go check it out. Dreamwidth has an entry and comment import feature (that I think is pretty spiffy: hats off to exor674 who did tons of work on it). When comments are imported from LJ, they are imported as OpenID comments, so you have control of them when signed in to your OpenID account. This means if any of your friends, acquaintances, or people you commented to once because they wrote something that you got linked to via metaquotes or metafandom are involved enough with Dreamwidth to be part of the closed beta, you may already have your OpenID profile partially set up and showing that you have left comments.
There is, as yet, no automatic tool to figure out who your friends on LJ are and if they've got a Dreamwidth account so you can add them to your reading list. Alas.
If your friends have come over, though, they may have already announced who they are in their own journals, and you can always check for their username, either by entering exampleusername.dreamwidth.org (replace 'exampleusername' with their username) or by picking the "Site and User" option from the search box (which is generally on the top right) and searching. (You can also enter someone's OpenID link there to find out if they've got an OpenID profile set up, in that same "Site and User" search bit. Enter azurelunatic.livejournal.com there for kicks, and you'll see me and my OpenID account!)
Dreamwidth has split LiveJournal's "Friends" concept into Subscription (Reading) and Access (you see locked stuff). (When originally thought up in the old days of LJ plotting, and when it was carried over to DW originally, it was called WTF -- Watch/Trust/Friend. People are still calling it WTF, because that name will never go away.) As an OpenID account, you are only able to add people to your reading list. There's nothing there for you to grant people access to see, because OpenID accounts can't post.
People with regular Dreamwidth accounts can grant you access, and can also subscribe to your OpenID account. Even though you can't post, you can change your userpic and stuff.
There's a News page. There's a wiki. There are some known issues, but if you run into any new ones, poke dom, or comment to the latest news post. There's even porn.
[Edit: For the love of Pete, once you get a regular invite code, don't delete the OpenID account. Unless you want to orphan any comments of yours that got imported under that OpenID account's control, or to possibly screw up linking the OpenID with the regular account once that becomes possible.]
("That's all great," my friends say, "but what are you planning to do, Azz?"
I am keeping my LJ right where it is, and posting happily away to it, and reading my friends page and commenting there. I am also chirping happily away on Twitter, and reading my people there, and importing my inane warbling to LJ. I am also importing my LJ to my DW account on a regular basis, and reading my reading page there, and commenting there. All three of these services auto-open in tabs when I start up Firefox, unlike Facebook, which I visit maybe once every couple weeks when I remember that it's there. Whether you're primarily on LJ, Twitter, or DW, I'm likely to be there. If we know each other and I haven't added you somewhere that you think I should have, poke me.
I never really got into IJ, so even though I have a journal there, I don't do anything with it.
I am still happily running suggestions and volunteering on the Support board. I also made my first touch on the Dreamwidth suggestions board this evening, and I was approved! Whee! When I have DW invite codes, I will let people know. )