I often use the iDisk feature on my MobileMe account to move files between computers, to servers, etc.
Unfortunately, I encountered some frustrating errors recently when dragging files to my iDisk. Despite different files and folders, zipping, and even re-naming them, I repeatedly got errors saying the copy could not be completed as a file already existed with that name. Eventually I realized I was dragging directly from a thumb drive (flash memory device) rather than my hard disk. The thumb drive was formatted in Windows-compatible FAT format. Turns out that iDisk does not like copying from that format, at least on a Mac.
Workaround: I moved the items to my Mac's hard drive (in HFS+ format). Then copied to iDisk without a problem.