Well, what do we have here? Turns out that sometime last year a Halo game for the Nintendo DS was being developed but for reasons unknown it was never published:

"Halo on the Nintendo DS? It could have been, if the deals had worked out. A major publisher began negociations with license holder Microsoft for the game to happen, and contracted a known game developer to produced a spec demo of a version of Halo for the Nintendo DSgameplay videos from the handheld. The demo was made, the game was (according to those who played it) appropriately accurate to the Halo experience on Nintendo's little handheld, and the talks were active in making the game happen ... and then, it was no more. Due to issues regarding the license, Halo DS was never fully produced."

Matt, over at IGN, posted up some gameplay of the handheld Halo. Looks pretty impressive. It's just a shame that it never went anywhere.