why not just use any one of the numerous backup programs to script a nightly copy to mobile me?
also, your mobile me iDisk mounts like a normal disk, to there is no reason you cant just treat it like a local drive, or more like a USB/removable drive.
I'd like to be able to leave my db file running. My understanding is that to use a back up program I would have to have the file closed, and that running the backup program on an open file might corrupt it. Correct me if I'm wrong on this. Closing it manually every day is not a viable option. I haven't used iDisk in a while, because when I've used it in the past the performance seemed slow. I'm also not quite sure how the syncing works in regards to the file being open or closed.
If you use Filemaker Server you could use its scheduled back up. When FMS backs up files, they can remain open. However, that sounds like killing an ant by dropping a mountain on it.
Look up the "Install On Timer Script" script option in the help file. It looks like you could write a script that uses "Save as" to save a copy of your file to a specified location and then use a 2nd script with Install On Timer Script to call it every 1440 seconds.