Raven and id both made Quake 4. While Raven did most of the content work and does engine level work, id was also doing game specific work for it. A similar situation exists for Enemy Territories Quake Wars with Splash Damage doing most of the heavy work, but id also helping out.

I'm sure Quake 4 will likely come to Steam at some point as well, and newer releases might come out at the same time on both physical media and Steam downloads.

Ironically, Steam wasn't working for the majority of the Quakecon attendees from the BYOC area, so for those of us at the event, we couldn't buy it, or even log in to look. Offline mode once again proves how not so offline it is. Maybe now with id also putting their stuff on Steam, Valve might work out a proper LAN party solution for Steam since id throws one of the larger parties.