I too came from the BB world and the mute functionality of the mic button on the headset was something I used all the time. On the BB it was short-press to mute/unmute, long press to hang up, double press to redial last number. Here's the rub - on a couple calls I was on last week, this is exactly how it worked on my (htc evo) 'droid (2.2), too. Today, it's working just as the OP stated, only as answer/hangup, and in my case a double-press will redial the last number. Does anyone have thier mic button working to mute/unmute the mic during a call?
I'll refrain from posting how annoying it is that when not in a call the mic button launches music player and plays the first song in the current list (totally eff'n stupid feature).
Sure would love to hear what others are experiencing in-call with wired, mic'd, and buttoned headsets...