If you want a continuous/ongoing sync, I have had good luck with DropBox. It's a free service that keeps folders synchronized on multiple computers. I wouldn't want to use it for anything super-secret, or anything very big. (I actually use it for my password collection, which is pretty safe because the password program PasswordSafe3 only writes files in encrypted form.)

If it's big, and you want to have some control over the process, rsync really is the best. The man page is intimidating, but skip down to where the examples are and there's some useful stuff there. Most often I use it like this:
rsync -e ssh /home/gconnor/whatever/ gconnor@hostname:/home/gconnor/whatever/

which basically means to sync from the local folder to the remote folder on "hostname" using ssh to connect and using gconnor as the username. The only thing that really trips me up sometimes and I have to be cautious of, is the / on the end... just be consistent on the source and destination so they both have the slash, otherwise you might end up with a folder-within-a-folder.

The upside to rsync is that you have more control, and you aren't trusting a third party like DropBox to carry and hold your data.

I have to admit that I haven't tried .mac/mobileme for its folder sync features... I use it for contacts, calendar, everything else but actual files. I am actually not sure if it has a feature to sync folders, or if it just offers some storage that's available from multiple locations. I'll be interested to see if someone else chimes in with more info about that.

Thanks!

Can they both access the same purpose-built time machine image?

I also use dropbox