You can record the Local Digital Broadcast channels.
This author is not familiar with MythTV but one configuration that is known to work is GBPVR with a QAM enabled tuner card such as the Hauppage HVR-1600
The following SW and HW is required for this configuration.
Software – The download for GBPVR and the install instructions can be found at http://gbpvr.com/pmwiki/pmwiki.php/Main/HomePage.
Hardware – Hauppauge WinTV-HVR-1600 model 1178
product code 74021
You must download the Version 3.4D1 QAM driver from their website as it is not included in the retail box.(http://www.hauppauge.com/pages/support/support_hvr1600.html)
The product code MUST be 74021 as other HVR-1600 product codes do not have the necessary HW on the board for QAM tuning.