Perhaps in the PC/Mac version? Please?
Matt Soell has posted a followup on the whole 'no filmz' issue in the Halo.bungie.org forum. It seems that it's not that Halo couldn't have the capability, but that it couldn't be finished in time:
The question is not whether films are possible, but whether they could have happened in a reasonable amount of time. The answer to that question turned out to be No.
In theory, we could hold off on shipping Halo until we'd added every feature we'd like to include - but by the time we finished, it's doubtful anyone would care anymore.
I haven't seen anybody talk about it anywhere else, but I'd like to suggest that recording capability would have been of far less use on the Xbox version of Halo than in the PC and Mac versions anyway. How many people record films just to watch themselves play? Not many. OK, perhaps you could make a recording and bring your friends over and watch them later-- but the real use was always to have a small file you could email somewhere or upload to a website so that any member of the community could watch.
How would you have done that with the Xbox? Even if Bungie wanted to add a film-swapping capability directly into Halo itself, the Microsoft online service that would need to support it doesn't even exist yet. What's more, it might not ever have been designed to handle such file-swapping, given that it could potentially be used as a vector to deliver malicious code.
Here's hoping that the extra time to do the Halo release for Windows and MacOS also means time to finish the film feature.