NPD's Household Penetration Study found a 3% uptick in Mac households for 2009. This makes sense! What's surprising (or not) is that of the 12% of homes with a Mac, less than 2% are Mac-exclusive.
The 2% figure is extrapolated from NPD less direct assessment:
[A] pproximately 12 percent of all U.S. computer owning households own an Apple computer, up from 9 percent in 2008. While Apple ownership is growing, those households are decidedly in favour of mixed system environments. Of those 12 percent, nearly 85 percent also own a Windows-based PC.
At first glance, these stats almost seem wrong, but when you start think about it, they make sense: The survey polled "households", which on account of grandma's Compaq or your roommate's gaming PC, clobbers the exclusivity figure. (I live in what any reasonable person would call a "Mac household"—three people who use Macs almost exclusively—but that little Acer netbook sitting on the table means we're not.)
Plus, PCs are cheap and they linger, and Macs, being pricier, tend to find their way into richer households, where more than one computer is almost a given. That, combined the fact that most Macs sold are laptops, and therefore a little more likely to be a supplemental computer, makes the 2% figure look a little less crazy, but still, 2%? Fanboys, you're slacking. [NPD via Macrumors]