I think doors nowadays are smarter with their security.  One thing I 
heard about is the control may have an embedded random number 
generator.  When you push the button, the door sends a number, then the 
control has to give back the right number.  This might be implimented in 
some kind of frequency-switching protocol.  With the amount of effort 
put into cell phones, it probably would be no problem for someone to 
develop come up with some low-power method.

--Alexander