if apple's push implementation is anything like Microsoft DirectPush or whatever they call it on Windows Mobile, then what the OP said is absolutely the opposite of reality.
DirectPush drains a battery faster because it basically has to maintain a connection to the tower all the time. We've seen battery life *increase* by disabling directpush and turning on regular checking at 10 minute intervals here at my company.
However, if apple does something where getting a push message is equivalent to getting a phone call or text message (as opposed to Microsoft's implementation of keeping a constant connection), then yes, battery life could be increased. I fail to see how it would be able to do this over Wi-fi, though, because with Wi-Fi you would have to maintain a connection all the time for push to work (a "pushed" request is not going to be able to go through a NAT'ed firewall without there being an existing connection, and hence draining your battery).
I think we need to see some real-world results before making assumptions like this.
Edit: My post sounds really negative. Actually i'm excited for push email, with or without battery changes for the positive or negative. Especially Exchange support...