Mapping a subdomain

I am attempting to map a subdomain url to a blog on my wpmu site.

Example: blog.domain1.com should point to blogdomain1.domain2.com

I've added a cname record to domain1 for blog.domain1.com to blogdomain1.domain2.com
I've also tried adding an A record pointing the the IP address of the domain2.com server.

With the A record in place I pinged both blog.domain1.com and blogdomain1.domain2.com and received the IP address for the domain2.com server for both replies.

When I enter blog.domain1.com in a browser I receive the Apache startup page.

Any ideas?