triplett: Just bought this yesterday during the Fall Sale finale and noticed the same thing - Serial Keys Depleted.
I don't know much about this game yet so have no idea what exactly the key is needed for and how much of a problem this might be. Maybe a Sticky FAQ to clarify this for new purchasers?
Under what conditions is a key required?
Just online multiplayer?
LAN multiplayer?
As VimesUK suggests, required by some mods?
I'm fine with single-player and LAN only, personally, so no key isn't a problem unless those play modes are affected.
From what I know you don't a need a key for single player or LAN play.
Edit: After clicking "Multiplayer" you're prompted for a key. Just click cancel and choose LAN on the left.