Hi,
we are running a tool on App Engine that allows users to sync GMail contacts between different Google / Google Apps accounts.
https://www.google.com/enterprise/marketplace/viewListing?productListingId=2083+12830481423215493060
We make a dedicated deploy to app engine for each customer (company) so that each client has full control over the datastore permissions etc.
Right now we have ~2000 instances running on GAE. A lot of these companies switched to Google Apps after evaluating our tool that gives them missing core functionality.
Whoaaaa pretty cool!