There is actually a way to do this fully in process. I think Rob Pike initially build this, and it's being used in the App Engine framework (or so I hear...).
Code: http://code.google.com/p/rsc/source/browse/#hg%2Fdevweb
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.