Google has now announced that it has expanded Google Sync to include availability for push e-mail for the iPhone and iPod Touch. Earlier this year, Google Sync was launched for Contacts and Google Calendar.
According to Google, new messages will normally be pushed to your phone within seconds, but this has a side-effect because push-connections uses more power than fetching at intervals, so you can expect lowered battery life, which has been discussed as a bug in the iPhone OS 3.1 in advance. Google recommends you to actually turn off push in settings, and fetch data every 30 or 60 minutes instead.
You can read more about how to set up Google Sync here
Read the entire announcement from Gmail blog