I had tried to setup Domain Mapping on a newly-installed Multisite setup. Everything seemed to be ok, except I was getting a server error when I tried to access the primary website which was setup as gositepress.com. Subdomains were working OK and I was able to login and map domains to them.
I finally found this in a PHP error log:
[06-Jul-2010 20:40:27] PHP Fatal error: Cannot redeclare dm_text_domain() (previously declared in C:\inetpub\wwwroot\wordpress\wp-content\mu-plugins\domain_mapping.php:29) in C:\inetpub\wwwroot\wordpress\wp-content\plugins\wordpress-mu-domain-mapping\domain_mapping.php on line 30
Once I removed the domain_mapping.php file from wordpress-mu-domain-mapping, it worked. Now, according to the installation documentation, I COPIED domain_mapping.php over to wp-content/mu-plugins. It seems, though, that it could be in either place but not BOTH.