Elantra/transmission vent

I have a 2006 Hyandai Elantra, I had the transmission fluid changed in Dec. 09 at the dealer. For the past month, i've noticed transmission fluid on the driveway. I brought the car to the dealer yesterday and they kept it overnight to see where it was leaking from. Today they told me it was coming from the transmission vent. They said it was overfilled probably when it was brought into some place for an oil change. I change the oil once in awhile, and then my wife takes it to the dealer. What would cause the fluid to start coming out now.