Make sure you delete all your table test data after running tests. In
RSPEC, this is done automagically for ActiveRecord, but not for
simple_record.
Make use of the prefix option in your configuration to create your
test domains (if you are not using domains under a different login).
Stub and mock whenever possible, really helps to keep test speeds up
aside from the traditional benefits.
What out for latency. SimpleDB has about a 1 second delay, in most
cases, between writing and committing (not sure thats the right term).
You can configure SimpleDB to automatically delay your reads or you
can sleep after writes or before reads in your own code.