I found that the AppEngine 1.4 SDK addressed many of these concerns. Personally, I've managed 50 requests per second on my blog without trouble, and with minimal CPU overhead. That's probably because everything is in memcache, so the database almost never gets hit. The pricing structure seems to actively encourage you to memcache as much as possible, too. Things are probably pretty different in a more write-intensive app, though.
I wrote my own. http://github.com/thurn/ackbar. If an article gets popular, I've paid up to $5, but below 6000 hits, it's all free. Latency could be better, but there's a lot of factors there (Clojure might be one of them?).
I take it that your images are rather static. If so, you should be adding caching headers which will cache them for free in Google's front end servers.
