Billy at Voodoo Extreme passes along a reply he received from John Carmack to a question about his mention of possibly releasing the Quake source code. His reply puts a time frame on when the source will be made available, which is after the first original Quake engine licensee has released their game:

"I can guarantee that I will be releasing the Quake source code, the only question is the timing. I don't intend to release it until all of the initial licensee projects have shipped. Anachronox looks to be the last one out, so pull for their completion..."
John Carmack