I venture to say that now that IE7 has been released you may eventually see some movement there. All sites of this size and complexity running off huge database backends were waiting for that event before upgrading code. And upgrading code on sites that large does NOT happen overnight.
Which is why I think it's majorly stupid of anyone to point to the big sites and use them as an excuse not to validate their little site. And there will someday come a day when quality of code does make some difference.