I feel the same way. It always seems that once I get to be known as the "shy" guy, for whatever reason I feel like I have to live up to that. It doesn't help that that if I do happen to do anything "out of character" such as have an actual conversation that people always make a big deal out of it like I suddenly grew vocal cords. It's almost as if I keep quiet just because I know talking will get me way too much attention from people who are accustomed to me being quiet.
This is pretty much my problem as well. For example, I made a joke at work the other day, people laughed, and then when they realized that I had actually spoke, it all of a sudden became this huge deal. They were like, "I didn't expect that, especially from you!" and other such nonsense statements. Then it just became this huge pile of awkwardness for the rest of the day.
It just takes me a long time to warm up to people and figure them out. Then by the time this eventually happens, I'm already labelled as "shy" and they just write me off and don't really know how to act around me. Blah...