I always thought it showed interest if you enquire about it. I would ring them.
With one proviso: as long as you call them after they said they would know. If they said they would have notified the applicants by (say) the 12th, and it's now the 15th, then a polite e-mail is just fine. If you contact them on the 9th it looks like you weren't paying attention, or are just pushy. Remember: even if you are the best candidate, it's probably close, so annoying them isn't doing you any favours.