>   Which mice are supported?

Anything which supports Apple Extended Mouse Protocol.  However, I
know of only Logitech MouseMan/TrackMan that do support the protocol.
For some unknown reason, each manufacturer seems to have decided to
reinvent their own wheel.

> How is the middle button emulated on a two button mouse?

With the standard Apple mouse and a keyboard, the middle and right
buttons are emulated by pressing the option key and left and right
arrow keys, respectively.  Logitech mice do not emulate the middle
button; they return each button's status in separate bit.

> On a PC (which I an forced to use at work) eXceed emulates 
> the middle button by pressing both buttons simultaneously.

You should be able to use a 3-button mouse from Logitech,
MouseSystems, or their compatibles on PC, too.  At least that was my
experience with NetBSD 0.8 with XFree86 (a long time ago).

For further information regarding Extended Mouse Protocol, my home
page (http://remus.rutgers.edu/~kenn/macbsd.html) has a pointer to
Apple's Web document.

ken