Google published a blog post with a screenshot showing how they might let you buy additional quota:

http://4.bp.blogspot.com/_dLfQMJsmsaI/SUg3NOeljVI/AAAAAAAAACA/Yh0ug378x6g/s1600-h/billing-blog-post-100percent.png

http://googleappengine.blogspot.com/2008/12/system-status-dashboard-quota-details.html

And today I finally received my invitation to App Engine. It's many months since I applied, but I'm now working on other things.