I don't have any data on this, but I suspect these live broadcasts might have the opposite effect, viz., encouraging people to go to the theater to see these ballets live. Wasn't that the rationale for companies all over the country staging Swan Lake, after people had seen the movie? Even now, people can buy (or rent) DVDs with Balanchine's Nutcracker or the ABT version with Baryshnikov/Kirkland. And last year, the cable channel Ovation had a Nutcracker marathon with many versions shown in December.
Having taken my 5-year-old great-niece to several ballets in the last couple of years, the live performances are irreplaceable. We first watch a DVD of the ballet with her so she knows the story and what to expect. But she is excited beyond belief at the prospect of seeing live dancers, getting dressed up to go to the theater, going out for lunch with her grandmother and great-aunt, etc., etc., etc.
I think the more important issue for regional companies is realistic pricing, as Colorado Ballet has done. It just makes so much more sense to fill the house, even with tickets at 30-40% off, and introduce people to the theater experience, as opposed to the crazy practice of closing off tiers at NYCB.
I hope your suspicion is correct. I do think that there's a difference between wanting to see a live "Swan Lake" after seeing "Black Swan" and wanting to see your own local company's version of "Swan Lake" after seeing a live broadcast of, say, the Bolshoi's. I'm hoping like crazy that HD broadcasts give non-balletomanes the fever to see dance live, but I am concerned that companies that rely on bread-and-butter warhorses to fill the house might see the audience taper off after a star-studded, "event" HD broadcast of one of the same warhorses blows through the local multiplex. (I'm also concerned that a newbie audience might find a well-danced local version disappointing for all the wrong reasons -- no stars, no big sets, less pyrotechnically thrilling dancing, etc etc etc, but that's a different matter.)
I'm not so worried about DVDs or television broadcasts because they aren't the same kind of "let's all go to the theater for something special" event that an HD broadcast is. There's only so much of that kind of festive energy to go around in a busy family's life, and if it gets channeled into HD events, there might not any left over for local live art.
Dirac -- I think our posts must have crossed in cyberspace -- as you can see, I too am worried about the disparity in production values ...