It's interesting that of all the various Google Data API's (which many use OAuth, I believe) Gmail isn't one of them. I guess that makes sense, since there are protocols like IMAP and POP... but for something like this it would be nice.

The Google Contact API supports OAuth, which is fantastic because so many sites want your email address and password to get contacts (for like invite friend style functionality) but it doesn't appear they store any kind of communication frequency data.