To me, a trackball is redundant to a touch screen (when combined with inertial scrolling gestures), with or without stylus vs finger.

Fast scrolling with a trackball vs fast scrolling with the flick of a finger/stylus
Click and Drag with a trackball vs press and drag with a finger/stylus

Finger and trackball are both fast and imprecise (almost every phone trackball I've used/seen is over-sensitive and doesn't offer a sensitivity control, so a slight slip of the finger throws you WAY off).

A stylus can make up for that, but having to get out a stylus is, IMO, awkward and cumbersome. I only want to use the precision of a stylus for fine point drawing, and maybe text selection. I don't want to use it for precision movement from widget to widget (which, in my opinion, neither a finger nor a phone trackball is suited for).

A dpad handles everything those 3 input methods doesn't:

• Precision -- When you move from widget to widget, you always do it ONE step at a time, you don't have to worry about accidentally slipping and jumping 3 widgets over (like with a trackball), or being unable to choose among 2 or 3 small text links or buttons that are close together (like with a finger). Whether it's precision of movement, or precision of clicking, the dpad wins over both the finger and the trackball.
• Convenience -- Unlike a stylus, you don't have to pull it out of its well, keep it on a lanyard, worry about setting it down and forgetting/losing it, etc. Depending on where it's placed on the device, it's always available, and you never risk losing it.

To me, the best combination is (all of these in one device):

1) Finger optimized GUI with inertial gestures for scrolling and such (flick your finger, send the scrolling list flying; have a slider thumb that appears only during scrolling so you can grab it and move it to the right place on the list (Android does this)).

2) A Dpad that is easy to use both when the device is closed and open.

3) The option/ability to use a stylus if you need to do fine point drawing, manipulation, or something similar. Preferably it could use the plastic tip of a pen cap, or something similarly common and easy to obtain (so that you don't have to worry about replacing one that comes with the device). I also like the Nokia Express Music "guitar pick" styli. But, again, it should be optional, and something you only really need for specialized applications. The primary pointer should be your finger(s).


So, as you can see, a trackball doesn't even enter into it for me. It's not necessary for speed, precision, convenience, anything (and, I still argue that it's not even _useful_ for precision). I find them clumsy and imprecise. Even after a year of using the one on my G1.

It might be acceptable to have an optical mouse in the middle of the dpad, like one of the latest Nokia phones (E73?). I haven't used an optical mouse on a phone yet, but it was a little bit awkward on the Raeon Everun Digital Note. It'd be ok, as long as you can turn it off and just use it like a regular Dpad IF YOU WANT. (that might give you the best of both a Dpad and a trackball)