Well, at this time you are probably fairly safe as far as the mac is concerned, but if you are in a lan with pc's you could pick up a virus and pass it on to them. It's not that there aren't any for the mac, but they are rare and you usually would have to help them along by giving permission for them to actually do anything to the mac.
As more and more start using a mac, then the chances are greater that someone will go to the bother of writing virus for them, but at this time the only real danger is passing a virus on to a pc. Mostly just use a little common sense and don't give permission for things to pop up and ask for your password to do things without thinking it over first.