OAuth has been used as Flickr and Twitter for some time, but is a new development for Google.
Previously, if you wanted to get push notifications on your iPhone when you received a message at Gmail, you had two options: trust a third-party application with your username and password, or automatically forward a copy of all of your email to a different email address and trust that they would not save a copy of your email. As you can imagine, neither of those made security-conscious users very comfortable.
There is already an iPhone app available which uses OAuth, SmartPush ($2.99) by Syphir promises to give you finer control over notifications from Gmail on your iPhone. We hope to have a review of SmartPush here on TUAW in the near future, so stay tuned for that.