I first ran into this while testing a new build of gparted under Ubuntu Natty daily.  I have since been able to replicate it under Maverick while running a kernel built from Linus's tree.  Sometimes it happens when removing a partition within gparted, sometimes it happens when I just run dmraid -an.  The call stack appears to be:

bdput
free_dev
__dm_destroy
dev_remove
ctl_ioctl
dm_ioctl

My guess is that something else is calling dput() once too many times and when you go to remove the device, it is freed early and triggers the BUG().

I'm starting to bisect now.