Gmail’s priority inbox feature became available for Gmail’s desktop web view late last summer. It attempts to deduce which of your incoming email is most important and tags it as important. It’s accuracy for me has been 50-50 at best. But, if you’ve found it more accurate and useful, you will be happy to learn that the feature has trickled down to Gmail’s mobile web view for smartphones whose browsers support HTML5.
FYI to Windows Phone 7 users: You are out of luck regarding Gmail and this feature. Windows Phone 7 does not support HTML5 (or enough of it).