iOS Gmail app. It's live now in the iTunes App Store for use with the iPhone, iPad and iPod touch. The new app offers a number of "time-saving features," including push notifications for new message alerts, quick message search and email address auto-complete from your Apple device's address book. The iPad version also offers up a split view, to read messages and check out your inbox at the same time. On the efficiency side, the Priority Inbox shows your important messages first, and the app also lets you sort messages via labels and stars. Swiping down will refresh the messages and swiping right gives you a quick view of your labels. Gmail will work with devices running iOS 4 and higher. You can check it out for yourself at the iTunes source link below.
Update: Google's confirmed on its blog that the Gmail app contains a bug that breaks notifications, and it's pulled the app while it fixes it. A new version is promised "soon."