I honestly think the Girlfriend Mode thing did them more good than harm. Seriously, you think anybody isn't actually going to buy the game just because of that statement? I mean they say that changed their mind, but I put that on the same level of BS as my friend the lifelong democrat saying Romney said WHAT? No way am I voting for him now! I think it's more likely that someone heard the buzz, looked into it, and thought "Hey, maybe this will get my girlfriend to play Borderlands 2 with me."
I wish Gearbox would just embrace the statement. Make it a running gag. Run some adds that make over-the-top humorous references to their "girlfriend mode" fiasco. I'd get a kick out of it. I also really want to see them have some fun at Diablo's expense. Unfortunately, I doubt either of them will happen. If I had the time and resources to make some mock advertisements though, I would just for fun.