When Google first started working on their Priority Inbox, they released a whitepaper on (a part of) the inner workings and sorting in the Gmail client.
As you might know, the priority inbox tries to learn what you think are important messages. Based on those learnings, it puts the most important mail at the top of your Priority Inbox, if you have this enabled.
How Gmail handles email sorting based on importance
Google released some more background in how they determine and rank the important (or priority of) e-mail. Importance ranking is a much bigger challenge than spam sorting. As users disagree on what is important, it requires a high degree of personalization.
Gmail tabs 101
Now with the Gmail tabs being introduced, it might be a good re-read and get some insight into “How Google handles their inbox sorting”. The whitepaper includes actual calculations and some key features. Download the PDF here: “The Learning Behind Gmail Priority Inbox”.