Natty desktop crashes at startup and hangs X

When I log into my normal user in Ubuntu 11.04, it looks like compiz is continually crashing and restarting itself - I see the desktop for a second, then application windows as they start up (without titlebars) for a second, then the desktop again... this continues for maybe 10-20 iterations during which time the keyboard has no effect other than if I switch to a tty console. Eventually X freezes completely and after that I can't even get a tty console.

I normally log this user into Maverick, and so I guess Natty is trying to boot with Maverick compiz 0.8 settings, which aren't completely compatible with compiz 0.9 - when I go back into Maverick, compiz has gone back to defaults. (It would have been good if Natty used a different path to store the compiz settings as it would make upgrades smoother.)

I have an nvidia card, so Natty is using the nvidia-current (270.30) driver, and I can log into a newly-created user without issues (well, mostly).

Anyway, given that X crashes and I can't do anything other than switch to a tty console while the crashing is in progress, what's the best way to get the crash information to try and figure out what's happening? Is there a known workaround that I can use?