You need to jailbreak your phone and inside Cydia you'll find "PushMod" which will allow you to set your mail check time to 1 minute instead of 15-minutes.
This is a great mod and has saved me a lot of stress and frustration, the lack of Blackberry style "free push service" makes iPhone less attractive, but this solves that.
Apple needs to make mobileme free and have a Blackberry style chat network to compete, I'm not willing to pay for something Blackberry........
First of all, the iPhone is crippled in many ways, but most of my complaints about functionality have been addressed through the jailbreak, Ican run apps in the background of my choosing, I can install a terminal, acccess my phone through SSH and SCP and so much more.
It even addresses the 15 minute e-mail problem, I installed a program called "PushMod" and now set the checking time to just 1 minute.
Apple should really thank the developers for this gift, it ma........