Chris Lanier has details of an beta plugin for Hauppauge HVR-1250, HVR-1600, HVR-1800 and HVR-2250 that enable unencrypted QAM on Media Center.
Hauppauge has taken touch from AVerMedia and created an unencrypted QAM plug-in for Vista Media Center. It is supported on the HVR-1250, HVR-1600, HVR-1800, and HVR-2250. Both Windows Vista x86 and x64 are supported as well, other information can be found here.
This is a beta plug-in, so as always approach with caution. You can download the plug-in here.
Hauppauge Releases QAM Plug-in for Vista Media Center - Chris Lanier's Blog