TeamXbox posted an interview with J Allard yesterday from the Xbox Unleashed event in New York. On the subject of online play, which comes to the Xbox next summer:

We are definitely ready for next summer. The first group of gamers to adopt the online play will be the really hardcore gamers. When Unreal Championship comes out, they've been working on that for 2 years, built from the ground up for Xbox, it will be ideally the premier online title, and that will be a total frenzy. We'll probably have 5-6 online enabled for summer, and then add another 12-15 over the year, so for Christmas 2002, we'll have about 20 online titles.

No mention of Halo specifically, but it's hard to believe that online play won't come to Halo in some form or another by next summer.