Isn't the act of developing for App Engine itself "developer lock-in", regardless of language? If I could write iPhone apps in Lua or Ruby I'd still be "locked-in" to the Cocoa platform. Same with writing programs for Android, Windows, Linux, Facebook,...
There are open source replacement for most of App Engine's services and API. The old language choice Java and Python are open source. GO is Google's language just like #C for Microsoft. Currently it's really easy for me to port my App Engine apps to AWS; I'm not sure if I use GO.
I believe AppScale supports Go, so I don't see how it's any harder to port a Go App Engine app to run elsewhere than porting Python or Java apps.
Go to Google is not like C# is to MS. Microsoft's C# platform is not free and they have complete control over the language. The Go compilers and libraries are open source. You're free to take and use them as you see fit (under the terms of its BSD license). Google can't "lock you in" to using Go.
