Read up on BlackBerry Connect (the second forum listed). Companies like Palm One have signed up to put the RIM e-mail software on their devices. So, you may one day see a Treo with RIM e-mail and a touch screen. One day. Maybe.
i have had the opportunity to try out my buddys Treo 600... what i noticed was that i hardly used the touchscreen or the stylus because everything could be accessed from the main keys.
Because the scroll wheel on the blackberry is so intuitive and functional, I don't think that I would even use the touchscreen if BB's had it. That's just my opinion though.
I agree with others who have noted that the hallmark of the BB above all other devices is it's physical robustness. A touchscreen would be just another item to break, poke, scratch, etc.
Having said that, RIM will probably give into demand for what might be perceived as an added-value feature.