I doubt this will work on a capacitive screen.If you're willing to install an Android build onto your sd card, there's a free app in android market called DroidScale that measures weight in grams and in ounces.
But it's not very accurate. If you're thinking of becoming a high-tech dope dealer, you're still gonna want to invest in a good old triple beam. And wipe that residue off your screen, it'll make your digitizer go numb and increase your cpu freq. til ur phone has a heart attack.
Sent from my... whatever