Here is a tip to Amazon - there should be a free implementation of SimpleDB. I am now considering using SimpleDB for a web application - but I would like to develop it on my desktop computer - without paying Amazon, without even connecting to Amazon. This might be a minor issue - the fee you would pay for developing the application on Amazon web services seem to be very small. But it would be so much more convenient: no fiddling with credit cards, no problems for developers with shaky internet connection and more compatibility with Free Software/Open Source development models where money is always a difficult subject.
For Amazon it would not mean much less income - since as I've said the fee for the development use of the services is rather small - but it would mean many more customers. Perhaps it would also mean some competition - but this would only validate the whole model and make it more suitable for big business which does not like so much dependence on one provider.