Q. I know that I'm supposed to send thank you notes after an interview. Can I get away with sending an email, or do I need to drag out my stationary and buy a stamp?
A. You can never go wrong sending a physical note. So few people do it that it's likely to make you stand out and put you into a positive light. Where speed is a factor or where the person interviewing you seemed to prefer a computer as a method of communication, then e-mail may be the right choice. However, the way you characterize your question "getting away" with something suggests that you're looking for the method that requires the least effort, and that is rarely the best way to impress somebody.
That said, e-mail is certainly better than nothing and does have the advantage of speed. The one time when you should absolutely go with a note instead of an e-mail would be when you have interviewed with somebody who gave you the impression that they were a traditionalist or that they had an appreciation for formality. In that case, you should demonstrate that you are well versed in all of the social niceties and send a nice physical note.