I am poised to upgrade to Pro…maybe even today.
But I’d like to understand the policies regarding installation on more than one computer.
I have three Macs and I also run Vista through Bootcamp and XP through virtualization.
What are my options for installing Unity Pro on different computers and platforms?
You can install pro on 2 systems.
A system is a distinct machine or a distinct os on the same machine.
Also, please keep in mind that you can’t keep Unity on the remaining machines license technically, as a single entity can only own either Unity or Unity Pro
As noted above, each license of Unity Pro is meant for use by a single user but that user can install on two different “machines”. Please note that installing on the same hardware but under different OSes (like Mac/Bootcamp-Windows, Mac/Parallels-Windows) counts as different machines.
But to be clear, you can choose your operating systems however you want. Two Macs? Two Windows? One of each? Other combo of two machines? Your choice.
Post up if you have any other questions as we’re here to help!
Oh, and before you upgrade go read this! 
If at some point I want or need to de-authorize one computer and authorize a different one—can I do that?
Yup, you bet! Just drop us a line at support@unity3d.com and let us know that you want to migrate your license from an old machine to a new one and we’ll walk you through it. 
HiggyB
thanks for your answers and your patience…
Not to be pedantic…but to be clear…
So if I initially authorize two of my macs (though only use one at any given time) I could later change and deauthorize a mac–and activate a PC.
Yup.
On my way to the store now…
I’d been waiting for the holiday sale the past few weeks!