the other side...
I'm going to be speaking from the side of a person who has been kicked out of a gaming group, and here is why... I was roleplaying my character to well, if you can imagine that.
The DM of the group was the type that allowed anything as long as it could be explained, it was Frealms, so I played a cleric of Talos, god of chaos. Unfortunately, he failed to tell me that everybody else in the group was lawful and/or good all around, so from the beginning my character didn't fit in. In the process of five weekends I almost killed the group seven times due to being a follower of this god, and at the end of the fifth week they all agreed to put the game on hold. Later, about three weeks later, did I find out from one of the players that they didn't want me around because of my character, and the DM didn't like the fact that I was playing this character that he accepted. Go figure, huh.
My point is that you must be very sure of your decision, and definately explain why you are making the decision. Sure, your going to probably apologize, but if the person is understanding then that's all the better. If the person isn't understanding, well, thems the breaks. But be very very sure that its the decision you want to make.