So here’s how I got instant email push on my 3G iPhone:

Here's my situation:
· I am using our Outlook exchange webmail to access mail at work
· I am using IMAP Gmail for my personal mail
With the way things were setup, I had to wait every 15 minutes for my phone to fetch my emails.
I could ask IT to enable some sort of push function for our exchange mail, but that would be perhaps difficult. Furthermore, Gmail does not have a push function for its mail yet. So following a path of less resistance, I decided to take advantage of push email by opening a free Yahoo account for email. I learned online Yahoo uses a P-IMAP function, which pushes mail when it's received.

Here's what I did:
1. Open a new Yahoo email account

(Sign up here: Link to Yahoo Mail)
2. Then set a rule in your Outlook to redirect a copy of mail to the above address.
(How to Article: Follow steps to configure mail redirect)
3. Then set your Gmail to automatically forward mail to the Yahoo account listed above
(Instructions: Gmail how to forward)
4. Then set your iPhone to accept mail from all 3 accounts

In your iPhone go into 'settings' from the main screen and set up 'fetch new data' as follows: Push needs to be 'on'

Under 'advanced' set:

i. Gmail to 'manual'

ii. Yahoo to 'push'

iii. Outlook to 'manual'
(Each mail site has well detailed help pages to connect to iPhone)
(More help can be found here: Apple iPhone Support)

Once you get mail from Yahoo, it acts as an alert for you to check your mailboxes, and on my phone usually takes about 10 seconds to show up the new mail in the other boxes once I initiate opening of my mail app. The down side is that you need to periodically get rid of the mail in the Yahoo account. I found the quickest way to accomplish this is to go into the Yahoo mailbox via the iPhone, and select the 'edit' button. Then check each message radio button, and then select the trash can. (kind of a pain, but hey, instant email)

In essence, my Yahoo account gets a copy of every message I receive. When it notifies my phone, I have new mail it is the copy of the message in the Yahoo mail. What I pay attention to when I check my mail is the number of messages that appear in either my Outlook or Gmail.