HomeLAN Fed's Halo PC Q&A talks with both Bungie's Michel Bastien and Gearbox Software's Randy Pitchford about the upcoming PC version of Halo. Here's a bit from Randy that helps explain why this port has taken so long:

We got the net code up to speed by starting over and re-writing it. Halo for Xbox was not designed to be played over the internet – we spent nearly half of our time on network programming and we think it’s going to be well worth the investment. What do you think? Do you think people will want to play Halo over the internet? Since we’ve gotten it running, it’s all I can think about…