Influenza and other viruses can live for as many as two or three days on nonporous surfaces, including your phone, the remote control and (no surprise) the toilet seat -- and some of these viral strains may live even longer. Bird flu, for example, can live for weeks, just waiting for you to have a seat [source: Wood, et al]. The common cold, on the other hand (or would that be the other cheek?), isn't that much of a threat. You could still catch a cold from the toilet seat, but rhinovirus usually survives less than a day on hard surfaces [source: Winther].
The trick with the cold and flu is not to touch your eyes, nose or mouth after you've touched the toilet seat (or a doorknob, or your office keyboard, etc.). It's easily transmitted through mucous membranes. We shouldn't have to remind you, but always wash your hands after using the bathroom.