I wish I had a thumb tackball. I tried to use one that had the ball in the center, but I did't like it. I need my scroll wheel. when I buy a new mouse I will defently have to see if I can find one of the logitech ones.
edit: I have one question though. Why would you need a cordless trackball? The only reason I could think of needing a wireless mouse is because the cable gets tangled up, and when you are moving the mouse around, it will only go so far, so you have to yank on it. With that said, a trackball almost never moves from its current spot, so I don't see what the cordless trackballs are needed for.