The reason I ask is because I may be able to get the game used for about $20 less.

Don't know, but I'd guess that they wouldn't want multiple people playing the beta off of one copy. So it's propably a code.
 
Also, double post. Herrp.

I think that the beta requires the disk to be in the 360 to work.

 @Everyones_A_Critic: its activated by the code but you also need the disk since the beta is a menu option once you redeem the code

Yeah, you get access via the code that EA packs with every new copy.  Once you redeem it, the Gears 3 beta option appears on the main menu (not selectable right now, obviously) so you will need your Bulletstorm disc once the beta is released.