vmWare Fusion guest OS display resolution broken (?)
I fixed this by, basically, screwing around with /Library/Preferences/com.apple.windowserver.plist on the guest copy of Mac OS X and getting lucky. First I tried removing that file and rebooting, but that didn't help. Next, I noticed that the 1680x1050 resolution was stored in that file, so I decided to edit it and change the numbers to 1440x900 (I figured if it's going to force a fixed resolution, I'll at least make it match the physical display) and rebooted.

The result is that the normal resize behavior is back. Changing the vmWare window size now updates the guest OS's screen resolution, as would normally be the case.

I don't know why this worked, but things are back to normal now.