If you manage to find a retailer selling the PC version of Borderlands early, you may be in for a rude surprise when you install the game and attempt to play it. Apparently, a number of threads started popping up on the 2K and Gearbox forums asking why their legally purchased copies of Borderlands could not even access the single player game yet.
Gearbox responded with the following:
Based on the post and the reply, it seems as though no parties involved are too thrilled about the situation, especially Gearbox.
I own a legal PC-copy of Borderlands, and we can't even play the Singleplayer cause of the shitty Securom-protection
Gearbox responded with the following:
The PC version won't be playable until the 26th - your retailer broke the street date 
Based on the post and the reply, it seems as though no parties involved are too thrilled about the situation, especially Gearbox.