As far as I am aware, the printscreen button is not assigned to 'screen shot' mode in Linux because as a button it can be used in console-only mode too, and a screenshot taken there wouldn't work! If you want to, though, you can get the keycode for it and make it take a screenshot. In fact, I think I'll do that now. Cheers for the thought
I use xbindkeys, so I just added an entry for the key with the following command:
import -window root "~/files/images/screens/screenshot-`date`.jpg"
and it works a treat.