It will also work in Parallels on a Mac, but the iSight will not for CUVA. X-Lite and other 3rd party SIPs are fine, but you can only have 1 per user...hence you cannot have a 3rd party SIP softphone and a Cisco SIP hardphone associated to the same user.
The CUPC client works well on a Mac (as long as it isn't Snow Leopard, you need the Beta client for that) but it does require a Cisco Presence Server to operate.