Were you using FileVault? Did you tell it to reclaim unused space?
Unfortunately, if you answered yes to either of those, things are not so easy to fix...
Here's how I fixed it the three or four times that it happened to me during the prerelease builds:
1. Copy your filevault (it is usually in the /Users/{yourusername}/.{yourusername}.dmg) to another drive - ensure you copy it to a filename that the finder will let you see (not a dot in front!);
2. Go to security and turn filevault off (seriously, it's borken, don't use it!);
3. Log back in to your account and open the filevault you copied elsewhere;
4. Check your files on the filevault archive. Generally, my mail was corrupt when it occured - and some of my prefs too - so be sure to check before you start copying stuff back into your non-encrypted home dir.
5. Cry. FileVault is fantastic, but the resizing of the archive (cleaning up unused space) appears to have not been tested so well. Hopefully Apple will fix this ASAP, otherwise there are going to be lots of angry mac users out there!
If you are using FileVault, don't use the reclaim free space option on logout. The home directory is contained on a sparseimage that can size up to accommodate more files, but cannot (of it's own accord) downsize. Personally, I'd recommend you turn it off until apple says they've fixed it.
This was a major reason I wasn't convinced that 7B85 wasn't final. I was wrong, but this is definitely a showstopper bug...