Domains mapping back to the same place

Hi, I'm _very_ new to WordPress so hopefully I'm not doing something crazy here.

I essentially just want to have two separate domains using the same WordPress installation.
Both domains will have different content.
The domains are boulderplex.com and 41pixels.com

I've set up Multisite, and the Domain Mapping and Multi-Domains Plugins as per the instructions.

The problem I'm having though is that both domains resolve back to boulderplex.com.

Under Sites, I have..
1.. boulderplex.com, Domain http://boulderplex.com, Path /
2.. 41pixels.com, Domain http://41pixels.com, Path /

boulderplex.com & 41pixels.com are listed under Multi-Domains.

As far as I can see the DNS is set up correctly. I've got vhosts for both domains set up in Apache and both these use the same DocumentRoot (/srv/www/htdocs/wordpress/ in this case).

I'm not sure what else I'm missing here.
I currently don't have anything listed in the "Custom Domain" (under Domain Mapping) for 41pixels.com. Could that be the missing step..?

Thanks!
Bryan.