Utilizing EC2 or Azure to scale out a distributed C# build system?
If you're using Git or Mercurial for source control, AppHarbor might be what you're looking for. It's a CI build/deploy environment that runs exclusively in the cloud (EC2), and can deploy build output to Azure.
Here are some links for reference:
http://sourcecodebean.com/archives/appharbor-heroku-for-net/987
http://lostechies.com/chrismissal/2011/03/12/using-appharbor-for-continuous-integration
http://haacked.com/archive/2011/05/12/making-let-me-bing-that-for-you-open-source.aspx
The open souce Jenkins CI server has an EC2 plugin that will spin up EC2 instances automatically depending on your build load. I couldn't find anything for Azure, but I highly recommend Jenkins - it's easy to configure, well maintained and has stacks of features.
Continuous Integration on Windows Azure http://code.google.com/p/cassis/ (over Mercurial)
Disclaimer: work produced by my 1st year CS students