I'd love to see a plugin/feature which does incremental backups and restore. A tall order I know, but it would be very useful if someone can build this as an automatic feature of Bukkit.
Use rsync or something like this. There's no reason to use a backup plugin. Maybe something like this for Windows, never tried that one though.
I'd rather not have to deal with disabling auto-saving in Minecraft and then jump back to rsync and then back to Minecraft, etc. It's a pain in the butt for me and I don't want to run the risk of corrupting my map due to bad timing with rsync. A plugin in my mind would be ideal to handle all of this within the realm of Minecraft.