wow! a new record???
only 4 posts into a "my mac just crashed without explanation" thread and we have someone saying it's probably "bad RAM." Every time someone has an unexplained kernel panic, the OSX-has-no-flaws-so-it-must-be-your-hardware--Have-you-installed-any-third-party-RAM?'ers jump in and declare the solution to be "replace your RAM!"
Not to pick on you, Mord, and I know you didn't yet tell him to buy more RAM, but that's usually the next step, regardless of what memtest returns. That or "take out your RAM and re-seat it."
I wonder if maybe the people who suggest re-seating the RAM have ever installed a stick of DDR? it's a tight fit, and it isn't just going to fall out of its socket. If anything, macs tend to have tighter-fitting ddr slots in my experience. my G5's RAM had my fingertips hurting for a couple of days because it took so much force to get the chips into the slotI thought I was going to break the RAM.
anyway, as SC68Cal mentioned, a crash log should help figure out what caused the problem. It almost certainly wasn't your RAM, though. Buy and run memtest if you like...by all means. it's only like a dollar or something.