I've had the same problem here, due to a "minor problem" with the volume header (on a MacBook Pro).

Booting off the OS X install CD will let you repair the problem (you can't repair the active boot volume), but here's where it gets interesting. Disk Utility claims success in repairing the volume header, but after restarting, the Boot Camp Assistant still complains of a disk error. Another reboot from the install CD finds the same error and (seemingly) fixes it again, but the Boot Camp Assistant still complains of a disk problem.

If anyone has any ideas, I'd greatly appreciate some help.