In my %APPDATA%\Bitcoin folder (in Windows 7) I have a file called wallet.dat.rewrite
It was apparently last modified on 12th May 2011.
I don't think it's a file I created manually, so what is it?
Sometimes the wallet file must be rewritten from scratch entirely (in particular when encrypthing it), to prevent unencrypted material to remain in old never-overwritten areas of the file. To do this, bitcoin writes a new file (wallet.dat.rewrite) and then renames it to wallet.dat. If this process is interrupted, it's possible that the wallet.dat.rewrite file remains. You can delete it safely.