Collision detection

I’m wondering how collision detection is generally done using AS3. I want to create moving and rotating MovieClips which update their own position using a setNewPosition() method, though how do i make MovieClips aware of eachother’s borders, so that once their border touch the movement of the movieclip alters. Also another problem arises with this. What if you have a movieclip which contains a 100*100 pixels .bmp with alpha channel. If that .bmp contains a complex form with around it 100% transparant pixels and one would write a collision detection based on the borders of the MovieClip then it would look like the MovieCLips bounced off eachother while not even touching. The MovieClip’s borders would be touching, but not the actual form in the bmp, because that form is surrounded by 100% opace pixels.
How would one generally go about doing something like this?