Yeah, do you mean that there is an option for gmail to push, or just that email is arriving in a push-like manner... ie. before the scheduled fetch time. 'Cause mine does that now on 2.2... gmail arrives at unpredictable times, usually right when it is picked up by my desktop mail client (which is before the 15 minute fetch interval it's supposed to be going by)
This is also the case for me. Gmail is Pushing and it does not have to do with 3.0. This worked on my 3G iPhone with 2.2.1. I tested by putting Gmail into Fetch mode every 1 hours. Each time someone replied to a MR thread that I was subscribed to, it updated to my email account and within seconds to my iPhone with a notification tone of new email.
I have also tested this on my Classic in fetch mode hourly with 2.2.1 and each email is arriving within seconds of the email account.
This is a Google thing, not Apple and I love it!!!