I think Forrester got it right - as they said, Microsoft is 'near-ubiquitous' in the large enterprise which this report covers.

When one of the enterprise NAC challenges is deploying a vendors big ugly NAC client, Microsoft clearly has the advantage with NAP and they have already surpassed anyone in terms of number of client deployments.

Cisco's NAC deployment assumes a Cisco Powered Network which is a non-starter for many companies.

I disagree with Forrester on one point which is that software based NAC will be successful. It's pretty clear to me that the last thing any customer wants is Yet Another Agent.