Hmm, I just tested this in 3.120 on Steam and it worked, then upgraded to 3.171 and it still worked. Is it possible that you were temporarily offline in the Steam client, or something of that nature? Did it say "Steamworks enabled!" in the top left corner of AI War at the time? Sounds like perhaps not, so in that case it didn't detect steam running for whatever reason; launching the game from Steam would solve that, if you didn't before. I'm on the June 15th version of Steam -- is that what you're on?
This is definitely a mystery, hopefully it's a transient thing or one of the above things helps.