Matt Soell of Bungie wrote in the HBO forum that Bungie decided they would rather write a whole new game with XBL support than revamp an old one:

Every person who would work on this hypothetical Halo 1.5 would not, by definition, be working on Halo 2. That would be bad. If we are going to hire a bunch of Bungie-quality bad-asses to work for us, we shouldn't dump them in the corner to rehash last year's product.

The amount of money we'd rake in from reselling the same game with a handful of quick'n'dirty new features is questionable...especially when everyone knows Halo 2 is on the way.

Bottom line: we'd rather work on something new, especially if we're going to charge you for it.

He posted on the Bungie Rumor Control page at Bungie.net about this issue specifically:

There will not be a Halo Gold or Halo Online.

We will not release a Halo game with explicit support for the Xbox Live network until Halo 2 ships.

Thanks Matt and HBO (HBO is a longtime Bungie fansite, but not the official voice of Bungie. Matt, on the other hand, is the official voice of Bungie.