Who should I ask for getting an API key for the GetPosts service?
You can get your API key from your profile page, appending GetApiKey to the Uri. You should find yours here:
I couldn't find a way to get that programmatically... and even then I expect you would need to sign in to LiveID first, which is not exactly easy to begin with.
EDIT: I'm not sure what would happen on a real device. You are supposed to be signed in to LiveID, but as that part is missing from the CTP emulator, this is just a wild guess.