Dolby Digital is the default format for HD but of course that doesn't always mean 5.1 audio
I am not sure how you mean "better 5.1"?
The standard DD5.1 format has various bitrates which 384/448kbps are the most often used, 640kbps is possible and used I think on some consoles.
There are new audio formats which vastly expand upon the well established DD5.1 format but these won't be used on broadcast sources they are being developed for pre-recorded media.
im hoping all HD films will be broadcast in their natural 5.1 states, extra nice would be if they do DTS too...heh....6.1 at that ;-)
otherwise why pay for HD, no point having HD picture with Dolby Pro Logic, kinda defeats the object in my opinion...lol
normal TV shows tho will still be Pro Logic at best for the most part, altho its possible i guess that some will be in DD5.1....i'm sure i've seen some TV shows on DVD in 5.1...