dstiles - 11:11 pm on Dec 22, 2010 (gmt 0)
On further investigation I discovered that Thunderbird (on Windows) no longer queried the URL [ubuntu.com...] and that the other URL I had replaced it with now feeds newsletters; previously, although it probably still did, I was only getting the security reports.
I installed [ubuntu.com...] on a Ubuntu machine, using Thunderbird, and the feed worked, so I tackled the Windows version.
On Windows Thunderbird I deleted both ubuntu RSS feeds and, after rebooting Thunderbird, successfully installed [ubuntu.com...] as the correct feed.
Rebooting Thunderbird is important: without that the feed verification attempt failed.