I too have had issues streaming HD quality movies from amazon. It always starts in SD. I have a wired connection to both my TCP-46G10 and DMP-BD60. Both behave the same. I did stream HD before the last vieracast update to my tv last fall. My internet routinely tests 6.5 Mbps.
I talked to Amazon today. They told me that the issue is with the last update from Panasonic. He stated that Panasonic has set a level of 7Mbps during the last update for HD content. He made it out that this was not intentional and would probably be fixed by a firmware update. I am not going to hold my breath though.
He stated tha that all devices really need different throughput to prevent buffering. He did state that Sony requires a lower level. He said like 6.
When I called Panasonic Customer Support they were no help and simply wanted to tell me it was not their problem and I should call my ISP. THey couldn't seem to comprehend that my internet could stream HD fine from my computer (TV episodes from Amazon).
Not sure how I am going to go forward. Does anyone have cable internet with an initial speed boost? It seems that this would fool the speed test that Viera Cast runs. NOt sure if it still wouldn't buffer though. Don't know if I want to switch from my DSL.
Maybe I will sell my panny blu ray and buy a Sony blu ray