It's probably too early to draw any conclusions, but still, I don't think it's gotten much attention: power numbers are down considerably throughout the league this year. Follow this link and go towards the bottom of the page and look for the "Juice Box."
Last year there were more than 1.1 home runs hit per game (which was actually higher than the rate in 2002 and 2003). So far this year there's been less than a homer per game (0.986). While in absolute terms that difference isn't so great, as a percentage it's down 12%.
ESPN has tracked these stats for the past few years, and last year - after steroid testing allegedly went into effect - there wasn't much impact on home runs per game, slugging percentage, or runs scored in aggregate. Sure, when you look at any given individual's numbers, there are bound to be variations year-to-year, but in total, there wasn't much in 2004 (the first year of steroid testing) that was different from what we saw in 2001, 2002, or 2003.
What's going on this year? Is it the cold weather in some places that's limiting power numbers? Possibly, but that's not that different year-over-year.
During spring training there were rumblings that "certain" (although never really specified anywhere that I read) pitchers had lost considerable velocity over the weekend and that was often linked to steroids (or rather the cessation of their use).
Maybe what we're starting to see now is that the steroid testing is having an effect. This will be interesting to watch.