if you are wanting to do it in FlashLite you will need to do the following:
make your button symbol, add a dynamic text field in it and give it the var
name you want. Buttons don't have independent timeline, so the varaible scope
will be the timeline that the button is sitting in.
Just be sure to turn off the field's selectability so that it doesn't
interfere with the hit state. I've uploaded some source for an example. you
can download it at:
http://www.infinitumdesign.com/mobile/dynButtonTxt.fla