I'm a Gmail user through and through. We even use Google Apps for Domains over at MoFuse, so it was kind of a downer when I got the new iPhone to realize that Gmail didn't have a push option like Yahoo or MobileMe.

I've been playing around for a little bit now trying to figure out a hack to get all my email pushed instantly to my iPhone and I came up with a workaround. It's actually not true push email from Gmail but rather a way to change the email address when you reply to an email (so it shows you@gmail.com rather than you@me.com)

You will need a MobileMe accounts, a Yahoo account might work but I haven't been successful yet.

Steps:

1. First thing you need to do is setup your MobileMe account and get it running on your iPhone.
2. Setup your Gmail to forward incoming messages to your MobileMe email address.
3. On your iPhone go to Settings->Mail->Your MobileMe Account->Account Info.
4. Click on SMTP towards the bottom.
5. Click on Add Server.
6. Enter in your Gmail (or other) SMTP server information here. Save and it should take you back to the SMTP page.
7. You will see at the top of the page the header Primary Server and right under it smtp.me.com, click this.
8. Turn Server to OFF, go back.
9. Click Account Info in the top left hand corner.
10. You should see smtp.gmail.com as your Outgoing Mail Server if you do goto step 11.
11. DONE.

Now when you receive emails to your Gmail account they will get forwarded to your MobileMe account and instantly pushed to your iPhone. When you reply to an email on your iPhone it will come from your Gmail account.

All email you send from your iPhone will also remain threaded in the Gmail web interface.

Screenshots:

smtp.me.com is set to Off

Outgoing Mail Server is set to smtp.gmail.com