I'm using a G4 PPC iMac and D-Link DSL-G624T router (via wi-fi), and I'm experiencing the same problem as the other used who pointed it out :(
When you start Transmission the port is both open and successfully mapped, but then when you open a torrent the button just goes red and the program is barely able to connect to peers..

I've used Transmission so far without a problem, it's my favorite BT client, I hope you can do something about this.. I'll still be using version 0.70, since that works fine, but I'd like to download the future updates as well.. :(

PS: Like other people, I'm also waiting for Transmission to be able to download single files from a torrent like Azureus.. That would make it the ultimate BT client for Mac :)