My company would require Windows 2000 support as we have no plans to migrate our 1500 desktops to XP en masse and thus can be expected to be in use for at least another year.

XP, Linux and Mac along with Vista...

And speaking about Linux and Mac...

Any chance to make just one version that would work on  almost any Unix like system?

Maybe by releasing the version in a way that could be relatively easy to link for either Linux or the *BSDs, by releasing the propietary code in the form of object files with the apropiate headers that would allow the users to link the software themselves.

1st. XP
2nd. 2000
3rd. 2003
4th. 98

Would block deployment:
Linux (1,000s of systems)
Windows XP (5,000) many dual boot Linux

Nice to have:
Other UNIX

Don't care:
Windows 2000 Pro and earlier

I think NAP should be made available for Windows 2000 and XP. If I only had one pick though I'd say XP definitely!

It must be avaible for Windows XP
Windows 2003

and NT4 of course ;-)

We should support NAP Client on XP and Vista. And we should have NAP Server on Vista(?) Server.
 
As far as non-windows OS's goes we should rely on free software developers to support these OS's. We can do this by publishing NAP protocol in detail and constantly reviewing our NAP protocol design to make sure it is open.

XP is a must, followed by Mac.  Linux is a nice to have.  

Like a previous poster noted, I view this as more critical for the client rather than the server.

We strongly need Windows 2000 support.
I believe most of all users in the world should have many of Windows 2000 computers.
The business strategy in the Microsoft will forget Windows 2000, however actual user IT environment is not so.

Win2K and Win 2003 Server should be supported.

In the large enterprize environment, Win2K is still main stream.

At a minimum Windows XP and 2003.  Businesses are not going to move wholesale to Vista/Longhorn right away. Additionally, support for Mac and Linux would be helpful.

For the Higher Education environment, we really need to have support for at least Mac OS X and common Linux variants (Red Hat, Ubuntu, Suse).  While it would be ideal if Microsoft provided these clients free of charge, at a minimum we need to have them available from a partner at a reasonable costs (<$5 per seat).  Without support for non-Windows operating systems, we'll never be able to force NAP client health validation in any widespread way.