I know that you can link a custom class to a movieclip symbol, by exporting it to Actionscript and providing the class name in the "Symbol properties" window. The question is: can you do the same with button symbols? A Google search returned a couple of pages saying it couldn't be done, and it didn't work when I tried it, but I haven't seen it mentioned explicitly in the docs, and in the "Symbol properties" window of my button symbols, the "class" field isn't greyed out.
(Not that it's a big deal, because I know you can always create a movieclip symbol and write a function for its "onPress" or "onRelease" event. I just wanted to be sure of this, to avoid wasting time trying it the next time).