Perhaps we're asking the wrong questions. Maybe it is true that Google never gave them access to their servers.
Has anyone asked if the government has demanded the TLS private keys? Could Google even answer that question?
As I said, I don't think secret MITM attacks are viable for many connections and large amounts of traffic. And AFAIK some browsers these days (Chrome for sure) have learned from the recent Root CA disasters in that they explicitly expect some "well know" site certificates (and google.com would be one of those), so they wouldn't trust any other certificate that's been been issued to google.com and signed by a trusted root CA.
IMHO this PRISM thing does what we always more or less suspected the NSA is doing -- it intercepts large amounts of public internet traffic, and automatically or semi-automacally tries to extract intelligence from it. A lot of SMTP traffic on the public internet is still unencrypted (even Gmail still accepts incoming SMTP deliveries without requiring STARTTLS or anything like that). So there's no really big surprise, but at least now we know for sure.