LifeHacker has published an entry on NoteSync, a note-taking application that syncs with Google Documents. It runs on Windows, Macs, and Linux.
An offline mode is fully supported. The program can be found here.
I have never used Google Documents; and I think I will stick with Simplenote for now, just because it does not work with the iTouch. Apparently, they are working on an iPhone application, however.