Yes, I know, but this is not the problem I was talking about.
I find myself implementing a caching layer for my models (this was pre-ndb) instead of the feature I intended to.
Be clever: write it once and for all.
This problem is nothing new in the world of programming. That's what libraries try to solve. GAE just happens to be a platform for which few abstractions exist because some of its technical choices render it slightly different than what the industry has been doing in other areas. If anything, this post is a scream for higher-level libraries targeting GAE.
App Engine is premium service -- it's replicated across at least three data centers, scales automatically, and has a world-class team managing it. They provide a free memcached service, and the pricing model is designed so that you use it. If you design your app to use memcached from the start, you can eliminate much of your read costs.