Ok, here's the deal ...
I stopped by my local Best Buy and had them network the 46 inch model and verified that the YouTube widget worked with version 1012 of the firmware. So we upgraded to version 1016 which is the (latest) version I am running, and verified that it worked as well. At that point, the only difference was that I was running the wireless dongle and they were hardwired.
I came home and tested with a wired connection and verified that it works. So, despite the fact that I have three bars of wireless signal at the location of the TV, and despite seeing successful communication with YouTube (lists of videos, video descriptions, etc.), the widget apparently does some kind of bandwidth/throughput check (?) and gives a video playback error if it determines there is insufficient throughput. Samsung? Verify?
Kind of sucks. I don't have a hardwired solution to the TV location. I'll have to look into running a cable to the location, boosting the wireless signal, running Ethernet over the power line, etc.