DynamoDB and SimpleDB are also a lot simpler than SQL databases. As far as I can tell, DynamoDB is a key/value DB with support for ranges and MapReduce, and not dissimilar to other NoSQL databases like Riak.
There may be instances where large datasets are hard to migrate from DynamoDB, but overall it doesn't look to me like lock-in would be that much of a problem, assuming you have a decent abstraction layer.
There may be instances where large datasets are hard to migrate from DynamoDB, but overall it doesn't look to me like lock-in would be that much of a problem, assuming you have a decent abstraction layer.