Joe Sheehan's article today is a good one, discussing regression to the mean in the context of C.C. Sabathia and Cliff Lee. However, when Sheehan throws out the names of teams and players that have gravitated back to the center since April, he includes one that doesn't belong--Barry Zito.
It's true that Zito's ERA is 4.78 since May 1. It's also true that his K/BB ratio in that span is 27/29 in 44.1 innings. Zito has had mediocre rate stats for years, but nothing like this. If you can't strike out more batters than you walk, you simply don't belong in a Major League rotation.
There's another lesson in Lee's hot start to the season, though. While a 0.67 ERA over seven starts doesn't mean Lee is suddenly a better pitcher than Sabathia, Lee's 44 K/4 BB and increased groundball tendencies over that span indicated a real change in his abilities. While his ERA is 6.04 in the five starts since, his peripherals have remained much better than the '06-'07 version of Lee.
Take early numbers with a grain of salt, but remember they're not meaningless.