If you've done this in the past, take the time to go back, take a look at the entry, and revise it if there are any changes. The basic meme as it stands only asks for your own personal basic information; I've chosen to include additional information that might be of interest to emergency medical types, including some medical history and contact information for individuals who are actually genetically related to me and local.
If you haven't done this, I recommend that you do. It's actually not all that uncommon for the closest friends of someone who blogs under a pseudonym to know a friend's username, phone number, given name, and deepest darkest secrets, but not exact physical address or family name, if those haven't been relevant for mailing care packages. iroshi has been close to me for almost as long as I've been on LJ, and has seen me at my absolute mental worst, and helped me pick up the broken pieces of my head and put them back together, and she still didn't know my family name when someone asked her about me. I hadn't thought these important enough to tell her, but these things can be relevant in the physical world.
The security aspects of this apply to all sites running the LiveJournal code. LiveJournal's Abuse team makes every effort to track down valid contact information and send help when they have a credible report of a user in danger of their life. I don't believe Dreamwidth has run into this situation yet, but Denise was the one to start the meme in the first place, so yeah. :)
Please post your full name, date of birth, full address, and telephone number (plus cell phone number, if you have one) to your journal, as the first chronological entry (backdated) in your journal, with Private security, and leave a note in your userinfo indicating that you've done so. The life you save may be your own. In the event of an emergency, LiveJournal administrators can view this information -- and please believe that it is only used in the event of an absolute emergency. But you'd be surprised how many of those there are on a service with four million users. And in an emergency situation, every second counts.
Yes, the Abuse team occasionally hears of users who plan to commit suicide, but there are other situations where having this information could possibly be of use to the police. There have been, for instance, phone posts that were interrupted by physical violence before, and other such situations. The Abuse team often gets reports of these things, and oftentimes, there is nothing we can do to help, because we can't find you.
Please take a moment of your time and make this post. Your privacy will never be violated except in absolute need, and though we hope it will never be necessary, it could prove to be very necessary indeed.
Everybody's comfort level with this sort of thing varies, but if you are comfortable with this, and/or have listed your contact information in a filtered post for sharing with your friends, consider placing one at the beginning of your journal where it's easy to find.