I had the same thing roughly 1 week after buying my brand new 2011 WRX.
Just took my car into my local dealer to fix and it hasn't happened again. They told me it was a burned out bulb, but I am not sure how much I believe as I couldn't see any burned out....
I was more annoyed than anything else that my brand spanking new car was doing this, I could care less about a burned out bulb lol