This is what I think happened based on everything I've read/seen/heard to date...
I think Zimmerman saw Martin and considered his actions suspicious, based on recent events. Said that Martin was standing in the rain looking at houses and when he (Martin) saw Zimmerman watching him he (Martin) put his hoodie on and started walking. I thikn Zimmerman called 911 to report it, and started following him. I think Martin at one point ran to get away. I think Zimmerman at that point got out and started following. Then, started walking back to his truck. This was all when he was on the phone with 911, and can be corroborated (as best as possible) by the audio.
I think (again based on all read/seen/heard) that Zim returned to his truck, or was in close proximity to it. 911 asked him for his address and he said he didn't want to say it out loud because he didn't know where the guy was (I guess fear could be guy might hear and thus know his address). Before getting off the phone, Zim asked the 911 op if the cops could call him when they got there so he could tell them where he was, then the call ended.
That, to me, suggests Zimmerman didn't know where Martin was, and was going to walk around looking for him. And this is why I don't believe that Martin was trying to get away. It would seem he had ample opportunity.
After the call ends, no one really knows what happens, but from everything I've seen/read/heard, I can honestly say that I think Zimmerman did in fact go looking for Martin. I think Martin, instead of running home or running away, probably hung around and was keeping an eye on Zimmerman. I think Zimerman and Martin (obviously) crossed pathes. That was when the "Why are you following me" and "What are you doing here" comments took place, followed by the altercation.
Yes, I think Martin initiated the altercation.
I can honestly say it doesn't make an ounce of sense to me that Zimmermann would give so much details to the cops, telling them exactly where he was, and knowing they were close, to walk up to Martin and start a fist fight. If you listen to the 911 calls, I believe that theory is supported. Zimmerman, for the duration of the call, is completely calm. No flaring tempers. No raised voice. He's calm the whole time.
I think, obviously, Martin was beating Zimmerman pretty good. And I don't think it's out the realm that Zimmerman feared for his life, and pulled the trigger.