Ralf's Sudelbücher has a great series of posts on SimpleDB. Though designed as an introduction to his Opensource, .Net based desktop clone of SimpleDB called NSimpleDB (short for .NET SimpleDB) it actually serves as an excellent introduction to SimpleDB itself.
Part 1 and 2 don't even deal with NSimpleDB but instead act as a nice high level introduction to the concepts and implementation of SimpleDB itself. In that capacity I think its worth everyone checking out. All that is required to understand those parts is a basic knowledge of relational databases.
As for NSimpleDB, I'm not sure I'll ever use it but I think its nice that someone made it available. With Amazon reportedly taking weeks to grant SimpleDB test accounts this seems to be the best alternative for .Net developers to get a handle on this new type of data storage.