Using Xbee to wirelessly connect an arduino project with a Unity game is possible. I have a video right here.
Now, how is it possible. Next up, look for Unity scripting tutorials (I assume a script is used to interface, and I know where the Unity scripting reference docs are). Also, familiarize myself with how exactly Xbee works. I was leaning toward Xbee over Bluetooth (both components are available through SparkFun) since I felt it might be more straightforward for my purposes. Now that I know this is indeed possible and not just my hopes and dreams, it is heartening. I’ll find it out.
Physical Computing, Round 2 