I have the same problem, but with Transmission installed in a Buffalo NAS and connected to a DIR-655. I also installed uTorrent and Transmission for Windows in a Win 7 PC connected to the router. uPNP is turned on and port forwarding for both clients.
I'm using a cable modem, with connections automatically going through a proxy assigned by the ISP.
Transmission reports that its port is closed. When I telnet to the NAS and run
netstat -nat | grep LISTEN
it lists the port, so I'm guessing that the port is open.
uTorrent reports that its port is open.
When I go to CanYouSeeMe.org, it reports that either port is closed.
I tried static IP addresses and manual port forwarding, and the results are the same.
Speed seems to be more consistent for the uTorrent installed in the desktop. For Transmission installed in the NAS, the speed goes up and down. I'm guessing that processor and memory limitations in the NAS makes this happen.
Finally, I'm guessing that the proxy of the ISP is blocking the Transmission port checker and CanYouSeeMe but not the uTorrent port checker.