If you needed to, you could solve most of the problems with SimpleDB by writing a server that wraps it and provides consistency checks, sorting and such. I was

If you needed to, you could solve most of the problems with SimpleDB by writing a server that wraps it and provides consistency checks, sorting and such. I was considering writing that for a little while. Then they announced that they were going to support mountable drives on EC2 images, and it stopped being necessary. When they release that, you'll be able to load up a more straightforward MySQL or PostgreSQL setup, and use traditional clustering mechanisms. Honestly, I find SimpleDB interesting, but I have no idea what I could use it for without writing quite a bit of middleware to wrap it.