Still suspecting Apple's servers, but I had an interesting experience last night.
First, I just replaced the main router on my network, the one that's plugged into the cable modem, replacing an old D-Link DIR-655 with a NetGear N900 (WNDR4500). I didn't think it ought to make any difference with regard this issue… and it apparently doesn't - but it made me willing to test iTunes streaming on Apple TV again, something I'm largely writing off for the time being.
I had the thought that Apple offers free episodes of TV shows, and that would be a painless thing to do: if it works, great - if it doesn't work, it cost me nothing. So I selected a show (the pilot for 'Go On', FWIW, which was in 1080p HD) and it started playing pretty much immediately and played through with no interruptions.
The last movie I rented, a few days ago, gave me an initial delay of 15-20 minutes (although it sometimes said it'd be an hour or two), and then when it finally said it was ready to play, stalled during playback 3 or 4 times.
Cool. Maybe the new router really made a difference? Emboldened, I immediately rented a movie I had been wanting to see, and… long delays again before starting. And hangs of several minutes each while playing.
Really? The free TV show streamed perfectly, but the movie that I paid for hangs?
In both cases, while trying to stream, I took Speedtest.net tests right beforehand that said that I had in excess of 18Mbps.
It's possible that the different programs were on different servers. But it certainly doesn't look like my ISP is throttling video coming from Apple, or that there's an issue with my network connection.