Sorry for bumping this but it seems this bug has been solved in xorg 1.8.1 :
It's been driving me crazy since ubuntu 8.10, it has been introduced into xorg somewhere after version 1.5.2, so ubuntu 9.04,9.10,10.04 don't work for me. Now I'm running 10.04. with xorg 1.8.1 from xorg-edgers PPA (which has this oneliner patch built in) for a week and not a single crash. After months of torture with twinview I can finally use 2 screens again
It's interesting how my bug on launchpad got changed from xorg to nvidia-drivers despite evidence to the contrary:
Thanks guys for pointing me in the right direction, towards xorg debugging.