Not sure why I never thought of this one before, but it's super easy to get notifications when someone uploads a file into your iDisk "Public" folder. You'll need Growl installed. I have my iDisk set up to keep a local copy on each Mac I use it on, so there is an actual file that then gets synced. I just chose the iDisk "Public" folder for Hazel to watch, with the following rules:
Rule 1:
All conditions
If Kind is Folder
-> Run Rules on Folder Contents
All condiitons
Date Added is after Date Last Matched
Date Added is Today
-> Send Growl notification with pattern "[file]" was added to your Public iDisk folder.
Not really rocket science, but if you never thought of using Hazel with your iDisk (as I hadn't), it's pretty nifty. Not sure how this would work if you only accessed your iDisk via the online disk.