The following is is a posting from the Official Gmail News Blog:
Posted by Marcus Foster, Product Manager, Google Mobile
Those of you who live in your Gmail inboxes usually want to know what’s happening with your email more instantly than standard fetch mail on your phone allows. Sure, using Gmail in your mobile browser gives you all the benefits of conversation threading and starring, but you still have to refresh every time you want to check for new mail.
When we launched Google Sync for Contacts and Google Calendar earlier this year, an over-the-air, always-on connection to sync mail was noticeably absent. We heard your requests loud and clear, and starting today you can use Google Sync to get your Gmail messages pushed directly to your iPhone, iPod Touch, or Windows Mobile device.
You can set up push Gmail by itself or choose to sync your Contacts and/or Calendar as well. If you’re using an iPhone, make sure you’re running iPhone OS version 3.0 or above (on your device, click Settings > General > About and scroll down until you see Version). If your software is out of date, follow Apple’s upgrade instructions. Then, visit m.google.com/sync from your computer for set up instructions. If you’re already using Google Sync, you can just enable push mail.
Once you’re set up, new messages are normally pushed to your phone within seconds. While this type of speed is pretty awesome, push connections tend to use more power than fetching at intervals, so don’t be surprised if your battery life isn’t quite what it used to be. We’ve done a lot of work to optimize power usage, but if you prefer to save battery life, you can always turn off push in your phone’s settings and fetch mail every 30 or 60 minutes instead.
The rest is here:
Push Gmail for iPhone and Windows Mobile