If you are using wordpress 3.0 currently (RC3) and want to setup different domains, with different ips on the same install, please use the trunk version of the domain mapping plugin, as the earlier version in the codex doesnt support it.
I have tested a wordpress 3.0 rc3 site with 10 different domains 3 with the same ips, and 7 with different ips, and it works fine.
http://plugins.svn.wordpress.org/wor...mapping/trunk/
here is the trunk version
