I'm sure the Netflix CS reps told you that, but as is often the case with CS reps, they likely were provided with an incomplete picture and are wrong. Unblock-Us is a service which has done a lot of testing and fiddling with the various streaming sources, and they have found that there is *absolutely* a difference at the Netflix server-side between the PS3 stream and AppleTV streaming methods. They use different protocols and have different connection and verification settings, especially with regard to non-US accounts. There's more details about it described
in this thread.
You've been bashing your head against the wall trying to figure out the problem. If you're interested in figuring it out, and maybe helping others with the same problem, then this is your best lead.
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Streaming devices that are officially endorsed by Netflix, like the AppleTV, Wii, and Xbox all use a separate protocol with separate settings and checks. This is
known and described in this thread.
It may be that the Roku isn't an officially supported/endorsed/whatever device by Netflix, and since Netflix didn't put it out, it simply hacks in to connect to Netflix via the PC/iOS/PS3 service.
I'd be interested in tracking down to see if it is the issue described. If you're interested, it may help to further a dedicated discussion
here.