I use windows live sync to synchronize one main folder (to rule them all) across all my machines. Then, from one of the machines, I periodically dump that folder onto a NAS. On the NAS I have a script running that does snapshot backups (like time machine) of that folder. Losing data from an accidental delete is far more likely than a hard drive crashing.
The only disadvantage is that the machine that dumps to my NAS has to be at home on my local LAN.
I am thinking of trying out Synology's DSM 4.0: http://www.synology.com/dsm/index.php?lang=enu
If it claims to do what they say it does, it should be like my own dropbox in the cloud. Then I can sync my one main folder across all my machines and my NAS. This eliminates the need for windows live sync - and I don't have to be at home to dump data to the NAS. I can also setup a script to make periodic snapshots of the folder. The only issue is currently the synology only has windows clients - and all my machines are macs... so I am still waiting to pull the trigger.