The NPD Group's 2009 Household Penetration Study, reveals that approximately 12 per cent of all US computer owning households own a Mac, up from 9 per cent in 2008.
The researchers note to that while Apple ownership is growing, the mix favors mixed system environments. Of those 12 per cent, nearly 85 per cent also own a Windows-based PC.
From the Press Release:
“Multiple computer ownership is a common thread in Apple computer households, with 66 per cent of households owning three or more computers, compared to just 29 per cent of Windows PC households. Apple owning households are decidedly more mobile as well, with 72 percent of them owning a notebook, whereas only 50 percent of households that have a Windows PC own a notebook.