I would presume that you could make the iDisk icon anything you wanted. Just like you can with any other drive icon wheter it is local or network mounted.
In case you don't know how to change the icon:
1.) find an icon that you like (google can be your friend here) and copy that image to the clipboard
2.) select the drive and cmd + i (get info)
3.) click on the graphic in the get info window then paste the new icon
A hidden file should then be copied to the drive letting every mac that mounts that drive see your new icon. Just is it can be done for folders.
If you still need the icons, I've got them. I backed up all the files that the MobileMe update changed (saw the updated ones in Pacifist). Place the files icons (in the zip) into :
System/Library/CoreServices/CoreTypes.Bundle/Resources
And then logout/ restart & your iDisk icon should be changed. I've got all of the old system graphics for .Mac still so, let me know if you want the rest as well. Maybe I'll put together a guide on how to restore all the old graphics