The only problem with that is say that your trust Microsoft Word Documents. That still doesn't mean they are safe because you can have Macro Viruses. Or you will always have an idiot who unzips a zipfile and runs that ub3r k3wl screen saver someone wrote.

I think a white list is a good idea, but it still doesn't make up for basic user education when dealing with attachments, updated client side virus scanners, and other precautions.