Many products rely on touchscreen input. Appendix A of Gregory Bender's dissertation provides an excellent overview of touchscreen technology, human factors and design research, with general guidelines. Note that this tends to focus on larger touchscreens applications (e.g. kiosks, ATMs), rather than smaller devices (PDAs, media players).
The appdenix starts on page 78 of this document, which is actually page 89 of the PDF file:
http://www.thisoldtractor.com/gtbender/papers/dissertation.pdf
