If you're thinking of using the App Engine read this first:<p><a href="http://highscalability.com/blog/2010/5/26/end-to-end-performance-study-of-cloud-services.html" rel="nofollow">http://highscalability.com/blog/2010/5/26/end-to-end-perform...</a>
some more thoughts on appengineJS here:
<a href="http://www.gmosx.com/blog/agVnbW9zeHIPCxIHQXJ0aWNsZRipwwEM/on-appenginejs" rel="nofollow">http://www.gmosx.com/blog/agVnbW9zeHIPCxIHQXJ0aWNsZRipwwEM/o...</a>