yesterday's post by Bungie staffer Frankie, what we already knew about Halo 3's mysterious X-button functionality was confirmed: It will be used to deploy equipment like the trip mines and bubble shields we saw in the recent Vidoc, and as of now is, appropriately enough, called Equipment. There's also some hinting about a third item that's yet to be revealed; we're hoping for nunchaku.
Frankie also dismisses worries of the bubble shield unbalancing gameplay as he emphasizes how rare equipment will be on the battlefield. So you can put your dreams of building a bubble shield fort back in your fan fiction where it belongs. There's also a heaping portion of the requisite undermining of all the video you've seen so far and reminders of how much better everything is in the game's current build, which will hopefully hold true when we see for ourselves in May.