If you’ve downloaded any apps for your phone recently I bet some of them use a paper texture for the user interface. The Twitter app and website spring to mind. It’s used to add a subtle richness without losing simplicity.
A quick search will give you this tutorial by an interface designer on how to achieve this effect in GIMP. It’s pretty simple but you’ll need to play around with the values to get the exact effect you want.
My iPhone-sized interface design is absurdly simple. There is only one place to click! It should be enough to demonstrate the paper texture though 😉