As many of you already know, Comcast now encrypts the vast majority of their channels. I am able to successfully use mythtv through a combination of clear-QAM tuners and a Comcast-provided Motorola DCT-6200 cable box connected via firewire:

• The following channels are transmitted "in the clear" and can be recorded with any clear QAM tuner:
• The "limited basic" standard definition digital channels (2 - 33)
• All local high definition channels (702, 703, 704, 705, 706, 707, 709, 712)
• The following channels are encrypted but can be reliably recorded using the firewire output of the Comcast-supplied Motorola DCT-6200 tuner
• All standard definition digital channels that I subscribe to that are in the range of 2-699.
  
• All other high definition channels that I have subscribed to, except 737 (TNT-HD) and FOX-HD (702).
  

The tricky part with firewire recording is that the station that provides the digital feed to Comcast can choose to activate and deactivate your firewire port on a per-show basis. This means that some shows from the same channel will record successfully via firewire and others will not. The specific problems I have encountered to date are:

• TNT-HD will not allow firewire-based recording of "The Closer".
  
• I "work around" this problem by only recording the standard-definition version of TNT over firewire. My experience to date is that the digital standard definition channels never prevent firewire based recording.
  
• FOX-HD will not allow firewire-based recording of "24".
• I "work around" this problem by only recording FOX via a clear QAM tuner.