Is it possible to wire up a cellphone screen to work with a different device?
One of the most popular items on Sparkfun is 128x128 Nokia Color LCD screen. The protocol has been reverse engineered by various people and libraries made for Arduino and other platforms.

In general, if the screen has only a few pins, it's likely to have a built-in LCD controller and use SPI or I2C for the protocol. Such protocols are usually not very hard to RE if you have the original hardware it worked with.

On the other hand, if it exposes some tens of pins like for example the PSP LCD, it usually requires a dedicated external controller and might even use LVDS or other complicated signaling. Such screens are not a good choice for small microcontrollers.

A lot of LCDs are available on eBay, see for example this store.