You can only use devices that can tune and demodulate the raw QAM256 signals, such as the HDHomeRun.
This does not include many channels on cable providers, as many of them (especially the HD channels) are ENCRYPTED, and require conditional access modules (CAM) to decrypt them.
In the case of CableCARD, one of the REQUIRED stipulations for CableCARD compliance, is a rigourous certification process which tests protected data paths within the decryption subsystems of a given reciever device. This means, that the underlying operating systems and their media stacks must provide working DRM in compliance with the standards that CableLabs has set forward, which have been approved by those that CableLabs answers to, namely, Hollywood, and the individual channel networks.
With the TiVO, while they are using the Linux kernel, their entire media stack is locked down by DRM, and can not be manipulated, patched, or extended in any way, the entire data path has been deemed to be "secure" by CableLabs.
But, you would KNOW this, if you ACTUALLY DUG IN, AND USED __GOOGLE__ FOR MORE THAN FIVE MINUTES. Everybody is affected by this. Windows, Linux, MacOS, doesn't matter.
These devices require an underlying operating system that does everything in its power to prevent the user from copying the resulting unencrypted stream, once it has passed through the CAM. A desktop Linux distribution does not have this, LinuxMCE does not have this, therefore we can not, and will not ever support these devices.
Please, do some research.