Someone noted before here, don't recall exactly who, in one of Steve's diaries that one should not increase a pitcher's load of IP more than 20-30 IP per year or risk blowing out his arm and/or harming his development. Someone on The Hardball Times take aim at that "rule of thumb" with some data - apparently Tom Verducci is also a believer of this rule and therefore disses Matt Cain: http://www.hardballtimes.com/main/article/the-year-after-effect/
He ran his study two ways, trying to answer any objections one might have over the population of pitchers encompassed in either dataset and came to this conclusion: that rule is a bunch of hooey.
Or in his words: "But the evidence points to the opposite. Pitchers who see a large increase in workload are more likely to continue to be successful than those who don't. It's important to remember that correlation does not mean causation--just because throwing a lot more innings than a pitcher ever has before is correlated with future success does not mean that managers should be riding their young pitchers hard--but it does imply that Verducci's argument is incorrect, and there is absolutely no reason that we should expect these YAE candidates to do worse because they've overworked."
That fits with what a number of us thought, that this rule didn't pass the smell test, but it is always nice when the numbers support you too! :^)