The Apple Store graciously reset my PRAM and firmware and whatnot with magic keypresses and suggested I try a complete reinstall with the original RAM. I did that and swapped machines with Cheryl just in case it crashed again when I'm off at RubyConf.
So I stuck one of my 1GB SO-DIMMs in her laptop and lo-and-behold it locked up! I've swapped the modules and am now 'testing' the second DIMM, but I'd be surprised if both are bad. At least I'll get my machine (and bag) back for RubyConf.