One of the first engineering things I needed to learn about was resistive vs capacitive touchscreens.
Synaptics had a white paper at Control Design that helped me understand the technology behind these options.
I am taking a look at several different products available in the market today including: iTouch, Garmin, Blackberry Storm, HP iPaq and other handheld devices I have designed recently that haven't been released to the market yet. I am constantly learning new things about what works and what doesn't work well with regular folks during usability studies.
There hasn't been much research on touchscreens yet, but the article on Evaluation of One Handed Thumb Tapping on Mobile Touchscreen Devices was helpful and it has helped me while planning for a formative study of the prototype early next year. In the referenced study, they found that preferred hand motions were quicker, that it didn't matter if you were walking or just standing while using the device, and that on-screen buttons on the edge had higher accuracy, but on-screen buttons in the center were deemed more comfortable to press. You will need to be an ACM member to download the full paper.