Medical student Kris Gunnars has a post on the topic, mentioning it’s a time-honored Indian folk remedy for treatment or prevention of dental problems.
I’d never heard of this before. I have no opinion on efficacy. Kris says to spit the oil out after swishing for 10–15 minutes because it contains “bacteria and nasty things.” I’d be tempted to swallow it unless I were concerned about the 180 calories in the oil. You’re swallowing those bacteria all day anyway. I imagine you have a mouth full of saliva after the first five minutes.
I’m filing this tidbit away for future reference.