Thanks for pointing me in the right direction.
After triple checking everything, and reading through the links again, it turns out my service descriptions were wrong. Apparently, the service description has to match the check_nt function name. That is, clientversion, uptime, etc. Once I changed this and set a freshness threshold, everything started working.
It sort of throws my naming scheme off a little, but hey, that's why I'm doing this in a test environment first.
Nagios is an awesome piece of software. It's been a steep learning curve for me because my *nix skills are rusty from years of NT system administration. I have to say, I love fedora, it's come a long way.