Sparkfun sells a bunch of LCD/touch sensor packages, some even pulled from commercial devices like the PSP. They can be hard to drive/use, but there are some popular ones that have a fair bit of hobbyist documentation on the internet. Here's an
all-in-one unit, and here is
just the touch panel from a psp, to lay over your own LCD. That second one also links to a touch screen tutorial you might find helpful. Note both of those are resistive, so no multi-touch.
Capacitive screens (like the kind that can do multitouch) are newer and more complex, so I'm not sure about finding them or info on how to interface them in the hobbyist realm.