This article gives a good primer on the twin banes of network television today: more ads, forcing the shows to run shorter and shorter, and more act breaks, forcing stories to be broken up into shorter and shorter sections.
The interesting question is whether timings can go any lower. I would be inclined to say that they can't go any lower than the current standard, which is two minutes of content for every one minute of commercial time -- but then, I didn't think they could go much lower than they were a few years ago, either. And it was only a few years ago that shows could have a tag scene during the closing credits (indeed, this was NBC network policy for some years, which is why shows like "Frasier" and "Friends" usually had closing-credits tags); then they started using the closing credits to show promos for upcoming shows. The old joke that they'll just cut out the program and show an hour of commercials is getting to be less and less of a joke every day.
One thing I find odd is that as timings have decreased, shows have become more heavily plotted. Back when a show had 24 minutes of content per half-hour, most shows had very simple plots and no "B" stories. Now that we're down to 20 minutes of content, we have shows with elaborate plots and "B" stories that try to cram everything they possibly can into this very short time.
Creatively, sitcoms are probably hardest hit by the short running times because there is absolutely no time to slow down and let a performer do his or her stuff. (You couldn't have Ed Norton taking all that time to write a letter, because you can no longer afford to waste that much time without getting back to the plot.) The miniscule running times could well be considered a factor in the decline of the sitcom; an extra minute or two of breathing space makes a lot of difference in a half-hour slot. Hourlong shows are somewhat less affected, especially serialized hourlong shows, because there's always the possibility of using something next week if you don't have time for it this week (this applies to serialized half-hour shows like "Arrested Development" as well). Still, an extra minute or two might make a lot of difference in the ability of some shows to get a little more creative and seem less like perpetual-motion plot machines.