Nice there's a note to the LJ community from the founder already posted.
Some highlights:
" I love technology and designing the LiveJournal architecture but I hate running a business. While I've been learning a lot of business stuff over the past 5 years and it's been kinda interesting, I just don't love it and I'm not great at it. Plus it just keeps getting harder as LiveJournal grows, sucking away more of my time and youth. I'm ready to pass off what I see as "the boring stuff" to somebody else that I trust and focus on the fun stuff.

Also, Six Apart has a lot of staff that we don't... marketing, designers, usability people, etc. "
Here's the press release stuff

CONGRATS EVERYONE

Update: Dave Winer says the motive could be a planned IPO.