Apple hasn't added push for Gmail to its Mail app on the iPhone, but Google has worked around that via Google Sync. All you have to do is set it up as a Microsoft Exchange account. At last!
Previously you could just sync your Google contacts and calendars, but now finally it supports Gmail as well, allowing you to be notified instantly when you get a new email. Push Gmail via Google Sync works on the iPhone as well as in Windows Mobile and Symbian.
Update: One thing to note: if you already have an Exchange account on your phone for, say, work, you won't be able to add a second for your personal account. It's never perfect, is it? [Google Mobile Blog via TechCrunch]