The company is urging developers to pay attention to concurrency issues such as datastore entity contention and CPU usage (initial free developer accounts are limited to 200M megacycles of CPU per day). Google also urges profiling application performance: The App Engine uses the Python logging module to view completed or real-time events and errors. But will developers heed these warnings?
The biggest problem facing developers is that pesky tangle called data contention, which can cause latency to mushroom in tandem with spiling traffic: reads and writes on a given entity are sequential. The App Engine datastore uses optimistic locking for concurrency control, according to the documentation: "An update of a entity occurs in a transaction that is retried a fixed number of times if other processes are trying to update the same entity simultaneously. Your application can execute multiple datastore operations in a single transaction which either all succeed or all fail, ensuring the integrity of your data."

The company is asking developers to post their thoughts on building scalable apps for the Google infrastructure. This space will be spying to see what the response means for concurrency in the mainstream.