I'm sure there are aftermarket options. First of all, I like the antenna. I think it is not only unique to BMW, I like the look. Second, it is a multi-function device. I'm not a technical expert, but I'm sure it functions as your AM/FM antenna also.
Not to mention, if you remove the shark fin, you'll have a hole and likely a mark from the base of the shark fin. Why deal with all that? With all that being said, my wife has the 128 vert. It has a smaller shark fin on the deck of the trunk, but because of the size, she still has a fixed antenna for AM/FM.
Current - 2009 Carrera 4S
Current - 2010 128i convertible (wife's car)
Previous - 2009 M-sport 335i coupe
Previous - X5 3.0i with prem/sport
Previous - 2000 ///M roadster
Someone should invent a sarcasm font...
"In car rock, paper, scissors 911 beats Prius everytime."