I posted a feature request on this...it would be nice to have adsense
feed directly into app engine fees, so it can all just run itself
without you bothering with it. Then even a third-world kid with an
OLPC could build a big site.
If the adsense revenues fall short, respond to any excess hits with a
donation page. For the kinds of sites that people feel charitable
towards, this could work nicely.
-d