I guess it might appear this way to you, but I'll just say that I watch a hell of a lot of TV shows (I just counted 15 major network shows in all), and I don't really get that "feel" from any of the uncancelled ones. Pushing Daisies didn't get a full season order initially because the network wasn't even sure at that time if the show would survive or not. Most shows do not get a full season order initially unless it's doing phenomenally well, but on average they all get as long a season as they like, which is usually up to 22-24 episodes.

Again, you might be seeing different things from the shows you watch (I'm not a Prison Break fan), but I get the opposite impression. I guess I'll go through my list of shows and see if any of them have a shorter run this year. I certainly haven't seen any kind of "trend" like you mention, particularly since it's hard to make such a claim when we just had the strike. Before that, I didn't see any shortening of seasons, and we haven't had a complete TV season since then.