Think about that--in just the last 9 seasons, the number of pitchers averaging less than an inning per appearance has nearly doubled.
Here are the total number of pitching appearances in those 9 years:
You can find the total number of games played right here, meaning that in 2011, pitchers averaged 3.86 innings per appearance (which includes starters) and in 2003, pitchers averaged 3.67 innings per appearance.
It would seem, though, that more individual pitchers are being used these days--more injuries or callups or whatever.
When I get more time and am not putting up a brand new blog, I'll go back and figure this by relievers only, which should be more illuminating.