Warning, this article is VERY MUCH a work in progress, at present it does not have complete instructions to install a touchscreen, though it will be updated as information becomes available.
Soon after the release of the eee, JKK of Jkkmobile modified the hardware to incorporate an aftermarket touchscreen kit into his eeepc. This became a popular topic, because it is a fairly inexpensive modification that produces very noticeable benefits.
There is some flexibility to the parts required for this modification because there are many different options available.
Touchscreens are simply a clear overlay that you insert behind the bezel and in front of the existing LCD screen and are touch sensitive. They are available in four-wire and five-wire variants. For JKK's build, he used a four-wire touchscreen. The touchscreen he used measured 169mm x 105mm x 1mm. It is recommended to find a Touchscreen Kit which will typically come with a USB controller and a CD or download of drivers. The controller should be as small as possible, though the absolute minimum size depends on where you choose to install the controller.