Catwoman #39 to be exact. Does anyone think this is really a surprise? Well I've seen some comments that suggest some are but it doesn't shock me. I'll be giving my thoughts on this and some other characters I've seen mentioned.
Honestly if anyone in the bat family were going to be bisexual Selina makes the most sense. I have no problem with this. I've seen some moaning over it and incredibly some suggestions that other characters could be instead. The three most requested being first 3 Robins. These are the reasons:
Dick Grayson: If you heard how the comic code came about you know why he comes to mind for a lot of people. The accusations of a less than innocent relationship with Batman that at one point lead to Alfreds' death and Aunt Harriet's creation. The reason being that having a butler was a fantasy lifestyle and they needed a woman in the house. Writer Devin Grayson related to the character (hence legally changing her last name) and saw him as a fellow bisexual. She even managed to slip in a bit with Hugo Strange determining Dick had a unrequited attraction towards the Dark Knight. Devin has said she sees Dick going for a man much like Bruce. This train of thought has been ignored in canon except for an odd comment by Barbara Gordon in the Grayson Futures End issue where she says he needs a woman to be just like Bruce.
Tim Drake: I think this is mainly because DC had him hardly mourn his girlfriends' death while he had a near mental breakdown over Conners'. Hardly mentioning Stephanie but trying to re-clone Conner and changing his costume to his colors. The fact Tim was a virgin (at least in the last 'verse) and the fact he didn't take advantage of a drunk Rose were other factors some took as proof. I never did especially since the first is a personal choice in general and the second is being a decent person. I'm not sure which one of these is the more popular choice with fans but this one I've seen more comments on.
Jason Todd: I think this started because Post-Crisis to Pre-New 52 he had maybe one semi-romantic interest, Talia. I've also seen a lot take his taunt of a man he hated (and in fact had poisoned) being taken as proof. Having heard Winick said something on the subject this is all I found when they were talking about the Talia thing: