There's been an ongoing discussion in the Halo.bungie.org forum about an item in the last update about Covenant units in Halo that would self-destruct upon dying, making killing them dangerous. Some sought to find links between similar suicide units in other Bungie games, like the Wight from Myth and the Assimilated Bobs in Marathon.

Case - aka Flawless Cowboy aka Jaime Griesemer, posted a small clarification on the issue, noting that the units don't intentionally suicide like the two aforementioned ones, and then added:

Man, I can't wait until you guys get your hands on Halo. There are things that nobody in the press has seen, or even guessed at.

While I suppose it's expected for a developer to want to hype his own game, it does seem clear that, despite all the information in Matt's updates, Bungie Studios has been able to keep enough secrets so that we'll all be pleasantly surprised when Halo finally hits the streets.