"Keyboard as the limiting factor on a PDA" - you are so right! Thank you for this great essay, Bob, you are speaking out of my heart.
One of the phenomena of today is that we try to minimize everything. The smaller the phone, the better; the smaller a powerful computer (i.e. PDA), the better. Everything should become more powerful, and yet smaller.
The problem is that my fingers would not get any smaller over time. If you happen to have my clumsy fingers you know what I am talking about. Today's gadgets are made for small kids who can write SMS messages on their cell faster that I can type letters on my PC.
SPB Full Screen Keyboard sounds interesting, but I am not sure if this would really enhance the speed of my typing. Another software for PPC I've heard of that should increase your input speed is
CalliGrapher.
I haven't had a change to play with yet, but perhaps you should give a try.