I think its very clear that the POLICE OFFICER was simply responding to a burglary call. It is in fact his job to ask for ID and make sure there isn't someone in the home forcing the homeowner to say everything is fine. At that point the homeowner, in this case professor Gates lost his cool and started calling a POLICE OFFICER that was bound by the requirements of his job, a racist.
Regardless of the pre-existing hatred the professor OBVIOUSLY has for the police and white men, if you lose your freaking cool when talking to a POLICE OFFICER, you're going to raise suspicions. Kind of like if you're speeding and get pulled over, it usually doesn't serve your best interests to flip out and start calling the POLICE OFFICER names and disparaging his mother. I don't care who you are, what color you are or where you are, showing disrespect to a fellow citizen who is doing a job most of us would be scared SH!TLESS to do for even a week is only a sign of YOUR OWN prejudices or racist beliefs.
As to why he was arrested, it seems fairly clear and supported by the neighbors accounts and the fellow POLICE OFFICER'S reports that Gates followed the Seargant out onto the porch and continued to loudly berate and insult him in from of the public. To any remotely honest person, this is the definition of disturbing the peace and it sounds like a night in jail is exactly what this unhinged gentleman needed.
If it was my house, same situation with a latino or black POLICE OFFICER and I acted exactly as Gates did, I would completely expect to get arrested.....wouldn't you?
If not I question your honesty and/or your intelligence