In adition to mice, stylii and touch screens, I've heard requests for an API web sites could use to get detailed touchpad data (rather than assuming they're identical to a mouse). In particular, when the hardware and OS supports it, it should be possible to receive co-ordinates for all the touch points on a trackpad.
The most interesting difference here is that the co-ordinate are not directly connected with screen co-ordinates. I could imagine having an additional pointer type where pointerdown corresponded to putting a finger on the trackpad, and there were additional fields for trackpad co-ordinates (perhaps in some fixed range like [0,100]).
I don't think it's critical that we need to flush out the details of this scenario now, but I'd like to keep it in mind as we define potential extensibility points.
Closed on WG conference call dated Dec 18th 2012 http://www.w3.org/2012/12/18-pointerevents-minutes.html#item08
RESOLUTION: Mark trackpad bug 20218 as resolved Later for PE v2