A lot of people have asked for touch, so we are considering how this might be achieved. It's possible we might be able to add it as a stretch goal or a "bolt-on" we just don't know yet. But enough people have mentioned it that we are sitting up and taking notice.
Update (18 Nov 2013). We've had a good look at both capacitive and resistive touch. Capacitive touch (like on your smartphone or iPad) would double the cost of HDMIPi, massively increase its complexity, delay its release and change its character completely away from being an affordable Hi-Def HDMI screen for the Raspberry Pi. We don't rule it out as a potential future project, but it's not going to happen in this iteration of HDMIPi.
Resistive touch is still under investigation. This may be available as a bolt-on. It's not nearly as good as capacitive, but it might meet some people's needs. We would only likely be able to justify it if enough people wanted it. We would also probably have to do this outside of the KickStarter project. We haven't got that far yet. Still need to see if we can make it work on the Raspberry Pi. This info will be updated when something changes.