As good as that sounds, it'll never happen. Chances are, the channels you do like demand the most amount of money in the first place So if they did offer a per channel plan, you'd pay a lot more than you'd expect. You probably have the top 20 most popular channels demanding the big bucks, whereas the 300 other channels are getting pennies per subscriber.
The business model sucks and is antiquated. More and more networks should be offering their programming online, paid or not. I'd get HBO or showtime in a heartbeat through an online subscription just so I wouldn't have to deal with my cable company.
The only problem is, though, as IPTV gets more and more popular, the cable companies will further restrict bandwidth caps and/or raise the prices.