We've just released an important new feature for Amazon SimpleDB. The new BatchPutAttributes function allows you to create or update up to 25 SimpleDB items at a time, in transactional fashion.
This new call is more efficient than a series of individual PutAttribute calls since one call incurs far less connection latency than a series of 25 calls. This new feature is available today and you can read about it here.
Update: Now that we've had a chance to benchmark the new batch put function in real-world operating conditions, we've found that it results in a doubling of the overall write throughput!
Also, based on feedback from the SimpleDB user community, we have increased the limit on the number of items per domain from 250 million to 1 billion.