Update: This news has been confirmed.
The latest murmurings from this weekend's QuakeCon suggest that id Software will be announcing a deal with Valve to distribute titles via Steam. Quakecon is currently going strong and lasts until August 5.
Should such a deal surface, it'd be interesting to see game engine competitors working together for distribution. However, since Half-Life was developed using a modified version of id's Quake engine, it's as if the Circle of Life is coming together.
Other rumors from QuakeCon include a return to the Commander Keen franchise as a flagpole for their id Tech 5 engine ... okay, we just made that up, but we can dream, right?