Getting Paypal's IPN to work with Ubercart
I had a terrible time getting IPN working for a client an it wasn't until I found a comment on a buried post that the answer came to light.
While everything worked fine in the sandbox, when switched over to live, no orders would complete. The user would pay, but then be redirected to the cart which would still have the order in it and the order status would stay stuck at "In Checkout."