hi:
does your touchscreen have circuitry or is it just the glass panel? If you only have the panel, I suggest trying to hook up the touchscreen to a controller and interfacing arduino with that. Texas Instruments makes a few touchscreen controllers and so doe Analog Devices. And you can order a sample from both for free.
Texas Instruments has the ADS7843 4wire touch screen controller that uses an SPI interface, as well as the TSC2006 which uses the TWI/I2C interface.
Analog Devices has the AD7843 and the AD7877. Both use SPI interface, and I think the difference is that the 7877 is a 12 bit resolution and the 7843 is 10 bit.
I recommend using the TSC2006 from TI because it is TWI and you can use the TWI library in arduino to communicate with the chip.
Also, read up on this article:
http://www.circuitcellar.com/library/print/0100/dahlin-114/index.htm
It is a great resource on resistive touch screens, how they work, and how to make them work.
Interfacing directly with the panel with arduino is also possible, but it is not as accurate as using a chip. Granted, using surface-mount chips is difficult, you will need to order a break-out board from sparkfun to solder it....
-nima