We already know a bit about Google's automated wiretap system for Gmail, because it was the system that the Chinese attacked in 2009/2010 in an attempt to access the accounts of Tibetan human right's campaigners.[1][2]
Bruce Schneier wrote a bit about this back in 2010[3]
I'm surprised no one has mentioned this before, since it's public confirmation of much of what the PRISM documents say, on the record.
Minor correction, it was not attacked to access accounts of human rights campaigners. It was leaked 2 months ago that Google lied about this and that the actual purpose was to see which of China's spies google were watching under NSL/FISA orders. So yes it was clearly Prism that they were accessing.
The breach appears to have been aimed at unearthing the
identities of Chinese intelligence operatives in the United
States who may have been under surveillance by American law
enforcement agencies.