I'm used to synchronize my data on different machines. Depending on the data I use SVN, Dropbox or SSH (Unison). Is this possible with the bitcoin wallet, too?
It is, with three provisos.
Never run the same wallet on more than one computer at a time.
Never run an older copy of a wallet when a newer version exists. If you send any funds, make sure to keep the newer copy of the wallet.
Make sure all client programs are linked to the same version of BerkeleyDB.
Overall, I think it's just not a good idea. The client software wasn't designed to cleanly handle the many things that can go wrong.
Yes. The only file you need to worry about is
wallet.dat (location dependent on operating system). If you copy that one file to another computer (or to a secure backup), you will have access to your money and address book. Conversely, that is the only file you need to protect -- all of the other files should be safe to share without worrying about anyone getting their hands on your money.
This only applies to your money and address book. If you only copy
wallet.dat, you will need to re-download the entire block chain on the other computer (so you may want to copy the other files too, but it is not necessary). There is no point in backing up the other files, because if you need to, you can redownload everything else.
Finally, you should not leave your cleartext wallet in any server that you don't control (such as DropBox). Make sure you encrypt it before backing it up.
You can get yourself an online wallet at a site like Blockchain. Then you can log in on the site from any computer to access your wallet.