I imagine it will definitely be released alongside a new engine or at least a major update of the Source engine. I'm not sure how much effect the success of Steam has had on Valve and their development of the franchise as I imagine a lot of their resources would have been tied to that as well as continuous support for titles like L4D1/2, Counter Strike and TF2.
Also have had the Portal titles released and now with Steam OS and machines on the horizon imagine even more of their resources are being utilised there so while it's fun to compare it to Duke Nukem Forever, with the level of secrecy at Valve it's hard to tell how long the title has been developed for, if at all!
EDIT: Let's not forget they also have the ever supported and updated DOTA 2 on the books as well!