The link varies per region, so go to http://www.greenmangaming.com/ and search for Borderlands 2, click Pre-Purchase and add this code: GMG25-1BW0K-K1A3G at checkout.
That should lower the price to $36.
The code expires the 10th.
EDIT: The code still works!
EDIT: Code lasts a bit longer.
25% off Digital Downloads with Voucher Code: GMG25-1BW0K-K1A3G
Offer Ends Monday 17th September 2012 at 11:00 UTC