Mythbusters #12, "Toothbrush surprise"
Basically, they did a month-long study. They put 20 toothbrushes at varying places within the bathroom, along with a toothbrush that was only used once put in the kitchen (under a cover if I remember right). Every day, they washed their hands, got the toothbrushes wet, and brushed their teeth with every toothbrush.
At the end of the month, they submitted all the tootbrushes (including the one in the kitchen) for fecal chloriform bacteria, and found it on every single toothbrush.
Nathan