Backing up your iDisk with Time Machine is easier than you might imagine. First, open System Preferences, select the MobileMe system preference, and click the iDisk tab. At the bottom of the window click Start in the iDisk Sync area. This will cause a local copy of your iDisk to be created on your Mac.
This local copy is stored as a sparse image bundle, which you can find on your Snow Leopard Mac by moving to your user folder and then following this path: Library/FileSync/incomprehensibly-named-folder where incomprehensibly-named-folder bears a name made up of letters and numbers. Inside this folder is the file you’re after – yourmobilemename_iDisk.sparsebundle.
Unless you’ve configured Time Machine to exclude your user folder and its contents, this is where you’ll point Time Machine to look for a backup. Note that when you travel back in time and restore this file, you’ll be warned that there already exists a file with that name and offered a choice to keep the original, keep both, or replace the original. I’d suggest keeping both. Here’s why:
I’m going to assume that people would want access to their old iDisk because they’ve lost something from it between then and now. If you keep the original, you can’t get to your old stuff. If you replace the current sparse image bundle with the old one, your iDisk and Mac are now out of sync and you won’t have the files you added since the restore point until they’re synced again. If you keep both, you have everything you need.
Move the restored file to your Desktop, double-click it to mount it, and grab the content you want. Leave the original where it was, remove the (original) that now appears in its name, and your Mac and iDisk can go back to syncing where they last left off