We are thinking about entering. We are already built on top of AWS. We have been incredibly happy about our uses of AWS (except SQS which was way to slow).
Definitely not counting on the challenge in anyway but could make for some nice publicity and give us good reason to post about some of the work we have been doing.
We looked at Starling but it wasn't as fast as Beanstalk. We ended up using Beanstalk and the Ruby client for our queues. I recommend it, but it doesn't persist to disk it is memory only, so if you have to guarantee jobs complete it not work for your situation. In our case if beanstalk goes down all the jobs are useless anyways so we can just restart.
Starling was a close contender, but it looks like twitter won't be working on it anymore as they are going to be moving to a different system. The beanstalk group is pretty active and has been very helpful.
Stomp also looks interesting, but we didn't try it out...
Definitely not counting on the challenge in anyway but could make for some nice publicity and give us good reason to post about some of the work we have been doing.