Many employers and I am sure dental schools, perform simple web searches to see who you are on the web. If you frequent forums and post really silly or bad things, look out. That info is out there in public and may come back to bite your ass.
That is why you should never, ever use your real name on most web sites, and why you should use an internet mail address that obscures your identity. It probably isn't very hard to figure out who you are based on a web mail address or even aliases in forums, but you can at least try not to make it too easy.
Avoid stupid stuff like putting pictures of yourself vomiting from too much drink on your facebook or my space page. Don't boast about how much weed you grew in college. Don't post nekkid pictures of yourself anywhere.
Stay off nambla forums, neonazi forums, etc.
Avoid getting into stupid arguments on forums, and don't call people names or insult them. Be civil and considerate in all on-line communications.
Don't surf for porn via either company or school servers. Companies log all internet surfing and have alarms that alert them to porn site access. I imagine many schools do the same. You don't want to suffer the embarrassment of having to explain what you were doing looking at a latex fetish site.
RP