Secondly a question :)
Play team solved the compile "problem" of java using a lib from eclipse so it compiles files on the fly. do you have something similar, or do you plan to include something similar? I think compiling on every change you make while developing an app is painful, the biggest problem java frameworks have, excluding Play! of course.
Here is a description of how it works: http://robfig.github.com/revel/manual/howrevelworks.html
Basically: In development mode, it runs a proxy (called the "harness") that watches your source for changes and recompiles/restarts your server on the next request, if necessary, using the "go build" tool. The go build process is incremental, so in theory should only recompile packages that have changed (and those that depend on them)
Code: http://code.google.com/p/rsc/source/browse/#hg%2Fdevweb
// Devweb is a simple environment for developing a web server.
// It runs its own web server on the given address and proxies
// all requests to the http server program named by importpath.
// It takes care of recompiling and restarting the program as needed.
out, err := exec.Command("go", "build", "-o", "prox.exe", rootPackage).CombinedOutput()
It's not what's used in the App Engine SDK. I believe it was inspired by what we do in the App Engine SDK, though.
This is a good reason to learn go :)