The distance of the home runs being hit this year (the true distance, i.e where they actually land, as well as the standard distance, which factors out weather and altitude) is significantly higher than last year, with the average standard distance being 8.5 feet longer this year than last.
The p-value actually works out to 0.0000341, which is a very strong indicator that something is making 2009 home runs fly farther than 2008 home runs, in isolation of the weather, and to me that implicates the ball.Ball speed off the bat is also up. And, as you would expect, so are home run totals. I will say that, as others have observed, the ball has appeared to me to be exploding off bats this year. I've been amazed at some of the balls that have gotten out at Yankee Stadium. Maybe my observations are being influenced by other people's commentary, but I thought I'd throw that out there. Apparently, this phenomenon is going on everywhere.
One of the most underreported stories of the so-called "Steroid Era" is that the spike in offense was not the gradual increase that one would expect if steroids were slowly permeating the game. It was a very sharp spike in the early 1990's (starting in 1993, if my memory serves; could be 1994). If we are to accept that steroids caused the surge in output, we must accept that hitters all started using simultaneously. This is highly unlikely. The much more likely explanation is that something changed about the playing environment, something universal.
The most likely culprit is the ball, and indeed there is empirical evidence demonstrating that the core of the ball changed and changed enough to cause a spike in home runs. It appears that something fishy is going on again this year. I'll be very interested to see how this plays out.
UPDATE: Apparently Brian Cashman reads the same blogs I do. ESPN reports that "Cashman also said home runs are traveling about eight feet farther so far this year compared to last season." Furthermore, Cashman is quoted as saying, "The ball is going farther in every park, not just ours." Fascinating.
**EDIT** Fixed typo noted in comments.