There was an LJ security breach. This post has the clearest and fullest explanation I've seen of how to tell if you were affected and what to do if you were.
I wasn't affected, but at least one person on my reading list was.
This entry was originally posted at http://aedifica.dreamwidth.org/659425.html. Please comment there using OpenID. The entry has comments so far.
Edited because "read this if you use LJ" works much better as a title on a Dreamwidth entry than on an LJ entry. :-)