We've had some good experiences building an app on Google App Engine, this first app's target audience are Google Apps users, so no issues there in terms of it being hosted on Google infrastructure.

We like it so much that we would like to investigate using it for a another app, however this next project is for a client who is not really that interested in what technology it sits on, they just want it to work, and work all of the time.

In this scenario, given that we have the technology applicability and capability side covered, are there any concerns that this stuff is still relatively new and that we may not be as much "in control" as if we had it done with traditional hosting?