That happening for anyone else? I can hear other people just fine, but I can't chat. I'm having a hell of a time diagnosing it, so I'm wondering if anyone else here was having a similiar issue.

I cannot speak specifically toward Borderlands 2, but since I've gotten deep into voice centric co-op I've found that having a Skype option really helps (if your not playing with pubs). A lot of games, even the steam client types ones, seems to run into random problems.

Dedicated VOIP stuff tends to work (so skype, or even a ventrillo server for a lot of people).

That said, have you verified your microphone is still fine (use the windows microphone loopback option)? One thing I found is that cheaper heatsets and desk microphones have very poor cables/contacts at pivots so they end up breaking down after a year or less. You can also run into issues if you have it set so that one program can take exclusive control of the audio device.

There's a push to talk button in game, are you using that? It's "T" by default.

yeah, of course i've been using the push to talk option, checked my mic (it works in skype, steam voice chat). I'll have to check the exclusive control option though, thanks @Zekhariah I forgot about that setting.

Well, that didn't fix it. I don't think its a common issue since this question is on the first page of google results...

@Riboflavin: Have you tinkered with the Steam voice settings? It's accessed in the options/settings. You can set the playback device (it might be using another "mic device" that you don't even know you have) and test if Steam is reading your mic there.

Beyond the ingame voice chat, you can also start voice chat sessions with individual people or groups through Steam's chat interface. Which you could test to see if the problem is in-game or with steam itself. (I kind of assume that Borderlands 2 is pulling voice chat settings from steam, but I could be wrong.)