What steps will reproduce the problem?
1. Place .htaccess in the blog's root directory.
2. Add the text "AddType x-mapp-php5 .php" - This will enable PHP5 rather
than PHP4.
3. Attempt to log in using OpenID at http://yourblog/wp-login.php
4. Replacing "AddType x-mapp-php5 .php" with "AddType x-mapp-php4 .php"
fixes the problem, but requires me to use an outdated, insecure version of PHP.

What is the expected output? What do you see instead?
I expect to be in the administration panel of my blog. Instead I get a 404
error with a bunch of OpenID stuff in the URL
(http://myblog/openid/consumer?...)

What version of the product are you using? On what operating system?
3.3.2 (newest)