Compaq makes a mouse with a trackball embedded in it. (as seen in an anonymous comment on the engadget Mighty Mouse story)

Mighty Mouse review roundup (thanks engadget)
Also, a Mighty Mouse review at ars
(thanks Roger) and one at GearLive (thanks technorati).

I’m still enjoying mine after a fair amount of use today. I had a little trouble with the shape - it feels less ergonomic than my usual work mouse(Microsoft Blue Optical). The ball is great, though, even without drivers for the horizontal stuff.

For the record, USB Monitor from HHD Software is incredible. It learned a lot about Mighty Mouse right from the start. The following observations come from moving and clicking while USB Monitor spewed 6 byte USB messages at me.

Byte 1: Shell down 01
Byte 2: Mouse Horizontal Movement
High numbers for left(FF-F2 FF for slow movement, F2 for fast movement, in between for other speeds)
Low numbers for right(00-08)
Byte 3: Mouse Vertical Movement
High numbers for up, low numbers for down
Byte 4: Ball Horizontal Movement
High numbers for right, low numbers for left.
Byte 5: Ball Vertical Movement
High numbers for down, low numbers for up.
Byte 6: The following are flag bits:
80 - Shell down
04 - Squeeze buttons being squeezed
01 - Flesh is touching the left mouse button area
02 - Flesh is touching the right mouse button area
10 - The ball is being pushed down

UPDATE:

A generic USB mouse(mousewheel: yes, optical: no) uses 4 byte messages according to tests with USB Monitor. Byte 1 is for button presses, byte 2 and 3 are for horizontal and vertical movement of the mouse and byte 4 is for vertical movement of the mousewheel.

The logitech mx510 sends 7 bytes. The first 4 act like a generic usb mouse, but the last 3 only change when the mouse is moving horizontally or vertically. I’m guessing they are related to accuracy. Does this mean that the accuracy of the Mighty Mouse is not as good as the MX510? Gamers beware, I guess.