All in all our 1st WordPress experience is going really well. We're finally stumped by this.
We're using IIS 7.5, PHP 5.2.12, and WordPressMU 2.9.2 in subdirectories with John's wp-404-handler.php set as IIS' 404 error page.
What's displaying, if we request a page that doesn't exist on the main blog or on a member blog, is the front page of the main blog. For example animals.com/nopage displays the animals.com front page and animals.com/dogs/nopage also displays the animals.com front page; the same for /cats/nopage, /horse/nopage, etc. (Adding any extension to nopage gets the same result.)
What we want to display is a custom 404 for each member blog. Each member blog theme does include a 404.php as does the default theme used in the main blog.
If there were theme errors, I would expect to see the default 404.php's "Error 404 - Not Found," but all non-existent page requests display the main blog's front page, not a 404.
Thanks in advance for any ideas.