How about those phones that can slide and open to show a physical keyboard, probably too costly and error prone I guess
I have a Nokia N86, I love it, and use it alongside my android "phone" (which is basically a tablet for my use case).
My ideal design would be an Android-powered Blackberry Torch format (with a slightly larger screen).
There is already a myriad of mini keyboards on ebay, although I haven't used any but some of them look sufficiently good to use along side a smartphone. In the future I think they will improve to the point where people who want a physical keyboard will just buy it separately.
Why don't you make a HN poll and find out? Link the poll to here. Smartphones a) should b) should not have a physical keyboard Or something like that!