This right here. Solved all the problems in the world. No more war. No more famine. Thank you sir.
I finally got Windows 7 installed (I had to do it twice actually, but that's unrelated to this issue).
The problem for me was corruption on my disk. I had orphaned inodes, extra file pointers (hard links), incorrectly reported free space, and one or two other things.
How I fixed it: Restart your computer, and hold down Command-R as it boots to boot into the recovery disk (I'm using Mointain Lion. I think Lion is the same, but on earlier OSes, you'd need to boot from the disk that came with your computer).
When prompted, open Disk Utility. Select your HDD and run "Repair Disk" which can take a while. Mine fixed the issues above. When it's done, reboot normally and try using Boot Camp Assistant again. Note that while it hopefully won't error anymore, the partitioning can take a long time. Mine took 1.5 hours to make a 40GB partition for Windows.
If you Repair Disk from Disk Utility running in OS X, it might work, but I'd recommend running it from the recovery partition, as Repair Disk is limited when running on the boot drive.