Due to the ongoing court proceedings between the previous heads of Infinity Ward, Vince Zampella and Jason West, and Activision the contract pertaining to latters deal with Bungie has gone public. This is definitely not how Bungie would like rumors on their upcoming game to be leaked, but that's how it goes. The full document was published by the LA Times, after the court made it public.
Luckily the contract spells out its terms fairly simply -- four games, four downloadable expansions, one of each every other year alternating (i.e. game in 2013, expansion in 2014, game in 2015, expansion in 2016, etc). The first game, codenamed Destiny, will be released in 2013 exclusively for the Xbox 360. The game will then be ported to other platforms one year later, around when the game's "expansion" is released. Further games will be made for next generation consoles.
Destiny, confirmed to be a sci-fantasy shooter, is also rumored to have various MMO features. That might explain the single downloadable expansion, as opposed to multiple downloadable map and content packs like Halo or Call of Duty.
The document also details a side-project of Bungie's, which they can only initially dedicate 5% of their staff towards. The game is referred to as Marathon, a reference to Bungie's cult hit from 1994 for the Mac.
No word on Bungie at E3 yet, and they definitely aren't listed on E3's website as having a booth. Whether they show up during a Microsoft keynote or in Activision's booth tour... time will tell. Expect Destiny news by July 7 though, when Bungie has its annual Bungie Day and celebrates with the culmination of their "One of Seven" contest.