Looking long term, I can see the trend Bruno is talking about. Star Trek The Next Generation for example was 26 episodes per season for it's late 80s to mid 90s run. Dallas (78-91) had seasons ranging from 22 (last season) to 31 episodes. Even Babylon 5, an unknown and risky show got 22 episodes for every season except the final (21).

What really annoys me is this trend to break up seasons into two pieces. Battlestar Galactica for example is not only airing in two halves (seasons 4.0 and 4.5), but is generally sold that way as well in both iTunes and on DVD.