It's been on Steam as a mod for a long time now (prior to the latest news update on their site, which was in 2007). I remember the devs complaining back in the day, because it wasn't meant to be listed as a mod, but as a stand alone game using the source engine.
I, like you Ned, was a massive fan of the original Alien Swarm mod and was quivering in excited antipication when they announced that they were working on the sequel, Infested.
The project has well and truly stalled though. It's amazing, if you look at the Black Cat forums, that people are still actually posting there, patiently waiting.
15 months with no word from the developers = dead game in my books. They were looking to be so close to getting something out too. It all looked so promising. Makes me cry even now, thinking back on what could have been... :'(
Warhammer Online - Darklands
Twahn - Knight of the Blazing Sun - 34/32 (TOG)
Karloff - Witch Hunter - T3
Tvarn - Iron Breaker - T2
Twahniel - Shadow Warrior - T1