I should have mentioned it earlier, but the Microsoft offerings are based around Windows Server 2003 R2 and SQL Server 2005. I wonder why this is not Windows Server 2008 and SQL Server 2008? Were Microsoft unwilling? Did it take a long time to certify the Amazon Machine Images and the 2008 editions are on the way?
James Simmonds mentioned to me earlier that he is using GoGrid. Looks like a good offering for Windows Server 2008 and SQL Server 2005. They are still in Beta but they are promising 100% uptime and 10,000% Guaranteed SLA... They are also showing some GoGrid vs Amazon EC2 comparisons.
I'd like to correct myself too: Amazon's cloud offering is not just on-premise software. Clearly Simple Storage Service, Simple Queue Services and SimpleDB are offering more of a pure cloud platform approach as opposed to the hosted image approach. Both will have a role to play.