It's difficult to find the information on the net because there's no law requiring it, and if there's a standard it's not publicly available from the general FCC and TV station websites.
Just a tidbit I ran across, a big tidbit mind you.
**broken link removed**
So it's not JUST horizontal. That information is a bit out of date though. By the information in that article it never occurred to me that they could transmit the signal in more than one polarization method, and I completly forgot about circular polarization. This is why I fact check everything everyone tells me =) And why antennas and the details of RF propagation are so hard to find out I think, there's so MUCH contradicting information out there.