how does the HD signal get to the TV???  

Not sure about the Avcast system but -.

1st - TIVO will not receive nor record HD signals. Maybe someday in the not too distant future this will change for the better.

2nd - viewing HD signals on a HD capable set of today requires 1 of 2 types of cables: RGB(HV) or Component (Y/PrPb). If this Avcast system re-broadcasts the audio/video on a specific channel I don't think it's going to work for HD signals. The only system that works or receiving HD using an OTA signal or sat signal is a HD STB (made by a number of manuf.)

Check out the FAQ list on their WWW site. The last FAQ says they are working on this for a future version:

The system looks pretty cool, especially once they support HDTV.

Prior to reception by the set top tuner box, HDTV over the air signals require a cable of at least UHF grade strung from the antenna through your home. I think the actual requirement is 900 MHz, you will have to double check the frequency assignments for each HDTV channel. HDTV via cable requries the same coax as the cable coming in from the street.

After reception and decoding by the set top box, HDTV requires a 35 MHz bandwidth component video cable set going through the walls to TV sets in your home. (For 1080i so few TV sets can reproduce the finest horizontal details that 25 MHz cable will usually suffice.)

Video hints:
http://members.aol.com/ajaynejr/video.htm