Look, I got an iPod Touch. I often tweet from it. In fact, I got over 22k tweets, 99% from various hand held devices, including iPod Touch.
I know for a fact that the virtual keyboards, its way too easy to make a mistake. To make a capital, you need to shift down, then press the key. To type numbers, I have to switch the ABC keyboard to the 123 keyboard.
I now have a Blackberry Tour. I am moving text faster than on my iPod Touch. I know, I can feel it. Less mistakes. To get a capital, I just merely press on the key longer. To get numbers, I press down on the alt key, then type on the designated keys to get the numbers with another fighter. I also get symbols that way. Nokia has a funny caps-underlining mode change on the key system that I feel particularly gets in the way of the typing. Not as intuitive as Blackberry, which gets the cap by pressing the key down a little longer. To get numbers and symbols on the Apple virtual keyboard, again, a mode change to change the keyboard. Not as efficient as pressing alt in one thumb, and the appropriate key on the other like you do on the BB.
To move cursor to edit text, finger tip isn't proving to be as precise as landing the cursor between two letters in the screen. I often have to delete the entire word and rewrite the whole word again since its faster. On the trackball, merely scroll the pointer to the precise space between the two characters.