I'm surprised it has taken this long for the subject to be approached again. I won't even bother dissecting the fallacious "but men play five sets" argument. I'm all for equal pay.
Based on what?
I'm not a fan of the "men play five sets" argument either. Nobody's salary is really determined by how long they work. After all, Tom Cruise makes more from 2 months of acting than most people will make their whole lives. In tennis, I'd pay more to watch Fed and Safin play best out of 3 than Kuznetsova and Shaugnessy play best out of 27. To me, the only thing that matters is how much revenue the women can prove they're directly responsible for generating.
If we're talking about Wimbledon, if they can prove that they hold just as much sway with ticket buyers, sponsors, and TV viewers as the men, then they deserve more prize money.
The "problem" is when people think of the "best," they generally think of men. When there's a woman's version and a men's version of a tour, the men's is generally considered to be premiere. WTA is by far the most prestigious professional organization for women, but when people think of the best player in the world -- not best MAN player or WOMAN player, just simply best PLAYER -- they think of Roger Federer. I think this also plays a part in it. People will pay to see the best.
In the US, though, Venus and Serena probably have more drawing power than just about anybody not named Agassi, Roddick, or Federer.