2000 Toyota Camry Gas Leak

I've noticed a gas leak coming from the gas tank area of my car. I've noticed it leaking on hot days after the car has been parked for a couple of days. The gas tank is relatively full - probably down 1/2 gallon from capacity. The leak seems not to be constant, but I've seen it dripping at a pretty fast pace.

I took it to the shop, where they determined the leak was coming from the charcoal vapor cannister which sits above the tank. I had them replace it, but noticed the leaking problem again today.

Any thoughts as to what the cause might be? I can say that the leak isn't due to overfilling the gas tank, and that the leak isn't overflow from the filler tube.