I believe what you really are looking for is a Distributed File System such as Andrew File Systemw
. This would sync your files across all of your computers, and create a virtual disk which would by shared by them all (and cached locally as you access each file).
I know people who use it a lot at the research unit I'm at, and it's extremely useful when you have several machines and lots of data.
I believe it also manages the replication of data, which would help with backups.
HOWEVER, from what I know, it's quite a pain in the *** to configure and manage. Expert computer science investigators have had quite some trouble with it
I'm just presenting this because someone might know a very similar solution that doesn't cause so many problems.
 I'm sorry if I'm suggesting something totally unrelated, as I only skimmed through the thread and didn't read all posts. If I am, please disregard this post