nicked from joyitude
Basically, LiveJournal changed it so instead of your journal being located at http://www.livejournal.com/users/pne/, it's now at http://pne.livejournal.com/, for example (with underscores turned into hyphens). Really not a big deal, especially as old links will continue to work. However, this does mean that those of you with underscores at the beginning or end, your journal will instead be located at http://users.livejournal.com/__username. (This is because domain name components aren't allowed to start or finish with a hyphen.)
You probably all are wondering why the heck I've bothered to tell you this, since most of the people on my friends list couldn't care less...this is why. From the news post: "You will not have to change your username, but if you'd like to change it we will offer you one rename (per username beginning or ending with a dash or underscore) for free." So if someone out there whose username begins or ends with an underscore has been wanting a different username for a while, now's your chance to get it for free.