Using mod_auth_pam with apache-2.2 causes strange error messages like [error] Internal error: pcfg_openfile() called with NULL filename.
Here's a patch that makes mod_auth_pam work with Apache 2.2.
You need to add the line:
AuthBasicProvider pam
to your existing Location sections where you want to use PAM.
The patch is made against the code after my other mod_auth_pam patch, so it may not apply cleanly without it.
mod_auth_pam-apache22.patch