Ubercart Amazon Payments will now tell the Amazon FPS API which IPN URL to use. This will be http://example.com/uc_amazonpayments/ipn. Also, a missing function parameter was added to _uc_amazonpayments_complete().
Now that Amazon FPS library for PHP has a -dev release, I'm creating one for this module as well. Try it out and report any issues in the issue queue. There definitely are bugs and things that need cleanup, but it also does work as a payment method.