I got the same problem with my sony Bravia set. HDMI does not work with HD DVR box. Verizon tech told me this is a problem with Samsung, toshiba, basically all the good HD sets. Tech also said they were waiting for a firmware upgrade to fix problem. Also, new 7200 STB should fix this once they release the box. I was very disappointed since I had to turn off the Media Manager features since it didn't work. Verizon should be more open about this issue. They also should not release a product that doen't work with most sets.
HDMI has been a problem with Verizon boxes since the beginning. All you have to do is use component (red/green/blue) cables instead. They work fine and the picture is equal to HDMI. With component you will need audio cable also, but it's worth it to have all the features work without problems.