SitePoint have written a really good article comparing Amazon EC2 vs Google App Engine with some great references

• google-reveals-app-engine
• http://code.google.com/appengine/
• app-engine-cant-handle-demand-for-google-app-engine
• app-engine-page-already-owned-by-hackers
• appjet
• http://www.amazon.com/gp
  
• scalr
• heroku