There was a bit of a hubbub on gaming.stackexchange.com recently, where some people thought links to gold-farming sites should not be allowed in profiles, while others thought they should. I took the stance that they should be allowed, but that appears to be the less popular opinion.
However, according to similar questions on meta.SO, and even according to Jeff himself, as long as there's nothing illegal or (too) offensive, profiles are pretty much anything-goes, including advertisements for your businesses.
So let me take it to an extreme: if I have a site selling male-enhancement pills, and I do everything by the book (have the right paperwork, don't sell where it's illegal to sell, etc.), can I have a link to it in my profile?