I noticed this morning that I now have this game in my games owned list, it does not appear to be any kind of free weekend promo, and as far as I can tell no one gifted it to me. I bought this game from Telltale, but not through Steam.  
 
Any one else notice this? Am I missing something? 

Weird. Must have checked to see if you had Steam when you installed it.

Figured it out I believe. 
 
Looks like if you register the code you get from Telltale in order to get the Team Fortress 2 rewards credited to your account it will add the game to your Steam account as well.  
 
If you bought directly from Telltale as I did, go here:  http://www.telltalegames.com/tf2visor 
 
Copy the code you get into the "add game" option in Steam, that is about the only thing I can think of that I did other than have the game installed on my PC.

Telltale's Alan Johnson says:

Right now, there are no plans to link a TTG purchased Poker Night at the Inventory to Steam. That said, you do get the pre-order visor via an email code that will be going out soon.