I had a conversation with my brother who runs prod ops over at <CENSORED> and we were discussing their use of EC2.  He mentioned that they don't currently use SQS to coordinate activities and instead they use a system based on a relational database for messaging and cited cost as a potential rationale.  I was curious about the break-even point so I made a little spreadsheet to summarize it.  You can find it here:

https://docs.google.com/spreadsheet/ccc?key=0Av3PKINeqTSBdF90Mk1ja2VXMExYc3JXVEJCUlY5TlE

If you assume you need 4 quadruple-high-memory instances it looks like you're better off using SQS  if your message rate is less than 8,000,000/hour.