We've updated What You Should Know about XHTML 1.1 to correct a few broken links and add pointers to (1) Anne van Kesteren's notes on extending XHTML and (2) Simon Jessey's method for serving XHTML properly with PHP. We'd appreciate hearing about any other articles that should be referenced.
Update: Patrick Griffiths points us to Declarations, his notes on defining valid XHTML documents. We should have pointed to his site -- htmldog.com -- sooner. It's a fine resource.