Personally, I wouldn't use anything less than 12px if it's primary content. Even at 12px, depending on the font family, those with a sight disadvantage are going to go somewhere else if they don't know how to adjust their browser font size. We're referring strictly to IE as all the other browsers are able to resize fixed font sizes.
With the higher resolutions being utilized these days, I've now been focusing on using 14, 15 and even 16px font sizes for primary content. When you've got a site showing at 1024x768 and it fills that screen nicely and the font can be read from more than 12 inches away, you're doin' good. ;)