IF you have "Local iDisk Sync" turned on (In System Preferences / MobileMe / iDisk Tab) then you CAN backup your iDisk using TimeMachine and probably already are...
"Local iDisk Sync" is where you can work in, save things to and delete things from your iDisk WITHOUT an internet connection. iDisk then SYNCS when it re-connects to the internet. This feature is OFF by default....
IF LOCAL IDISK IS TURNED ON then your iDisk and it's contents are stored in your - Home / Library / FileSync / (Bunch of numbers) folder in a "your users name"iDisk.sparcebundle... THIS IS YOUR LOCAL IDISK AND IS A COPY OF YOUR ONLINE IDISK (STORED LOCALLY)...
If timemachine is backing up that folder then you ARE backing up your iDisk and it should be by default...
NOW, if local iDisk is turned off then you are NOT backing up your iDisk locally and your ONLY copy in on Apple's servers... You HAVE NO BACKUP and are a very trusting person... Just sayin...
I hope this helps