Priority Inbox started as a "20 percent project" at Google Zürich, back in 2008. It's been initially called Magic Inbox, a name that was referenced in Gmail's source code last year. Google wanted to hide the complexity of the feature by using a simple interface that can be customized to suit your needs. The "+"/"-" buttons that let you change the importance of a message were inspired by the street signs from Zürich.
Ario says that Priority Inbox could be improved by grouping related conversations, so you can quickly deal with multiple messages. It's just one of the many improvements that could make Gmail's inbox smarter, especially for those who receive a lot of messages.
{ via Piers Fawkes }