Jeroen Sangers reports that Movable Type's latest crack at stopping comment spam -- a URL proxy script designed to limit the effect unwanted messages have on search engine rankings -- generates un-encoded ampersands, which render XHTML layouts invalid.
There's a simple solution, which Jeroen provides:
I did a quick search in the new files, and found the error in the file lib/MT/Template/Context.pm at line 1189. Just add 4 characters, and MT produces valid HTML again.
That is, replace the ampersand at line 1189 of Context.pm with &, or &, its numerical equivalent.
Update: This error has been fixed.