Lets see how those sites are using XHTML then. I'll confess I didn't check the content type of every one of them, but I only noticed one being served as application/xhtml+xml.
I also overlooked syntax errors. No center element in Strict? If its well formed, then it will pass the XML parser, so I won't complain about it here.
Of the others only 5 looked like they would function if treated as XHTML instead of tag soup (and I only checked the front pages, there could be errors on the inner pages).
The others were either not well-formed, so would cause XML parsers to throw an error, or commented out their JavaScript in an effort to protect Netscape 2 and co from rendering it by accident, so that it would get discarded by the XML parser.
Which makes about 80% of the "examples of XHTML not being a failure" being examples of pages that would fail if not treated as tag soup.
Wikipedia - not well-formed
Twitter - actually seems OK at a glance
Wordpress.com - actually seems OK at a glance
Blogger - comments out JavaScript so it would be hidden if treated as XHTML
digg - not well-formed, comments out JS
TechCrunch - not well-formed, comments out JS
Comment is Free - comments out JS
upcoming - not well-formed
Facebook - not well-formed
TypePad - comments out JS
LiveJournal - comments out JS
43things - not well-formed
Grazr - not well formed
37signals - not well formed
MeasureMap - actually looks OK
TUAW - not well-formed, comments out JS
Gizmodo - not well-formed, comments out JS
Skype - comments out JS
Adobe - not well-formed
BBC Backstage - not well-formed
TheyWorkForYou - not well-formed, comments out JS
Mashable - not well-formed, comments out JS
Rocketboom - not well-formed
Ze Frank - not well-formed
Jeremy Keith - served with correct content type!
Zeldman - not well-formed, comments out JS
A List Apart - actually seems OK at a glance