Blood Purity in Slytherin
Are muggleborns ever sorted into Slytherin? I've read several assertions that, canonically, muggleborns are not accepted into Slytherin (which seems to make sense, given the OotP Sorting Hat song about how each of the founders selected their students). All of the canon muggleborns I can think of are in other houses. Clearly, the Hat doesn't insist on total blood purity (or else Tom Riddle would never have been sorted to Slytherin, and it would never have offered Harry a place there), but what factor does it play?