Not known, though the only thing they actually announced was push through exchange. (Gmail would be through IMAP IDLE, presumably, which some people argue isn't "push," anyway).
I don't think GMAIL even supports IDLE yet. In fact, I think GMAIL's entire IMAP implementation is still pretty raw (i.e. needs a lot of work). The label -> folders thing, for example, is madness. I had to remove all my labels just to stop my iPhone and Mail.app from downloading multiple copies of each message.
GMAIL does support IDLE. I use it in Mail.app to read google mail, and it definitely works.
Yahoo Push uses some other thing - not IDLE. I forgot what it was. Proposed new spec. There is some discussion of it by Marc, the Chattermail guy, elsewhere on these boards.
