I think there will be more than one dominant device. Someone will want big phone, someone will prefer small laptop. Or maybe something completely different will come, something like rollable ebook reader?
if you are US-based, get out of the country to learn more about mobile computing.
phones are getting smarter, and now their OS are now a lot more accessible, laptops on the other hand aren't getting much smarter, but rather smaller.
if you go to Korea, Japan, India, several South American & African countries, you will understand why mobile is a heckuva lot more important than laptop.
it's not a UI conundrum -- a mobile phone (with the exception of the iPhone)'s primary purpose is communication -- voice, sms, mms and now even photo & video.
of course it will be a locked device like the iphone. the mouth-breathers don't care, they just like shiny. they'll gladly impoverish themselves for what passes as a jewelery fad in 2008.
I am one of the people who thing that's the next big thing, but I just can't see what will be the dominant device.
Will it be something like a smart phone/iPhone, or a just tiny yet powerful laptop. I think its mostly a UI conundrum.