HBO also notes (thanks again Louis Wu) that if you don't like the GameSpy matchmaking interface, there is an alternative that some like better, although it isn't free. It's called The All Seeing Eye, and it supports CounterStrike, Quake 3 Arena, BF1942, and now Halo. There's a 21-day trial and the price is $10 per year or $30 for a lifetime (assuming the business lasts as long as you do; the service has been around now for over 2 years).

Reppyboyo mentioned that Halo support had been added in the Gearbox forum; there's no official note on the All Seeing Eye's site; in addition, you have to set your default scanner in All Seeing Eye to the Finland 2.0 beta test scanner to see Halo servers.