If you have "local iDisk" turned ON then the contents of your iDisk is saved on your Mac as a "Sparse Disk Image Bundle". It is found in your home / library / filesync folder and therefore time machine would be backing up your iDisk (though for most, not in a useable way)...
If you do not have "local iDisk turned on" then you are depending on Apple to keep your stuff safe...
There are workarounds... I use SuperDuper to backup my local iDisk to a partition of an external drive every night. This has saved me a number of times when iDisk goes haywire...
Hope this helps...