You can do this in Ruby, and there is some argument whether it is a good thing to be able to do (in terms of readability, comprehensibility). You can override an existing method w/your own (I forget what the term is, right now, mainly because I never use it). Ruby on Rails takes full advantage of this.
Apparently, you can do the same thing in Flex, for the internal functions/capabilities of the MX Controls.
Void where prohibited, your mileage may vary, etc.