I fought the same problem for a week. First I read and attempted the fixes that worked for others to no avail. The I got on the phone with Apple, and they suggested some of the same fixes, but not as thorough as this forum. In this process, I fully restored the ATV2 twice via iTunes, unplugged it untold number of times, changed the Dolby setting, changed the HDMI cable, changed the HDMI out setting, tried a different HDMI input on the TV, and generally banged my head against the wall. Ran speed tests from my PC through ISP and was getting 7+ Mbps download speed. Failed to mention I am using a wired ethernet connection to the ATV2, not a wireless, but in the testing I connected it to the wireless newtork instead to see if that helped but it did not. After all this, still got the error loading content if I tried to watch a Netflix video, tried to send a video to Apple TV from my iPad2, or tried to watch a video (HD or SD) from already downloaded to my shared iTunes.
So I went to the local Genius Bar, laid it all out, and without any additional discussion, they gave me another ATV2. Brought it home, set it up. SAME PROBLEM. Repeated all the steps above, no help. Then I noticed the 10/100 5 port ethernet switch I had in the cabinet sending internet to a variety of devices, including the ATV2. I disconnected the feed from the switch and plugged it directly into the ATV2, and without even unplugging/rebooting the ATV2 the problem went away. Everything worked. Despite my speed from the ISP, the switch between the router and the ATV2 was slowing it down too much. So why didn't the wireless work?Well the wireless access point I was using in the living room was feeding off the same switch.
Hope this helps someone.