The actual feature will not be supported until AT&T enables MMS and these other features on September 25.
Although Apple hasn't released push for Gmail to the Mail App you can get around it with Google Sync and set it up as a Microsoft Exchange Account. Unfortunately if you have an Exchange account already on your iPhone, you will be unable to make a second one. (Source: Gizmodo)
This works great for me! I'm running Windows Mobile and already had my Gmail account syncing with Calendar and Contacts, I just went in and checked Email and it works perfectly. I'm really glad Gmail is doing this!
It initially didn't work on my iPhone. I got those "incorrect password" errors. I think it was because I was adding a second account on the iPhone to the same Gmail address. After removing both versions of the email account from my iPhone and then adding it once the Microsoft Exchange way, Push Gmail has been working great.
Sort of works on my blackberry curve 8900. I can login and get contacts to sync. Last time I tried it a month or so ago it was doing bad things. E.g., deleting a contact via the web would pull the, now missing contact, from the phone and vice versa which was very lame. Saw some unconfirmed funny business with the calendar and decided to uninstall it. The one thing it did that I liked was chew up more bandwidth. With ridiculously high unlimited data rates I try to get my moneys worth!