It supposedly costs about half a million extra to film, produce, and transmit a half hour sitcom in HD. These networks are going to be fighting the FCC and Congress a long time hoping they don't have to go full-time HD for many years.
Everytime costs go up, they always come to the distributors and ask for more pennies. NFL contract goes up, so does ESPN's fees. So if they do go all HD you know as sure as the sun rises so will your bill.