Regarding your concerns about Cisco Network Admission Control (NAC):
NAC will work for customers running operating systems from companies other than Microsoft. NAC 2.1 includes a Linux Cisco Trust Agent (CTA) that works with Red Hat Enterprise Linux (several versions).
NAC uses the CiscoSecure Access Control Server (ACS) as both its authentication server and policy manager. ACS can be configured to use a variety of different authentication sources (besides the built in Radius capability). ACS can use LDAP to talk to a variety of different authentication sources.
NAC was developed to be far more than a "Microsoft-Cisco deal".