I looked more closely and it looks like the link I posted previously applies to the server edition of OS X. The regular version requires the same steps, but is more manual. You need to manually edit the httpd.conf file (in /private/etc/apache2) by uncommenting line 465: "Include /private/etc/apache2/extra/httpd-vhosts.conf". Then add virtual host information to vhost.conf (in /private/etc/apache2/extra). Here's an example of virtual host info copied from Apache's website:
<VirtualHost 10.1.2.3>
ServerAdmin
[email protected]
DocumentRoot /www/docs/host.foo.com
ServerName host.foo.com
ErrorLog logs/host.foo.com-error_log
TransferLog logs/host.foo.com-access_log
</VirtualHost>
Restart Apache by turning web sharing on/off in System Prefs and it should go.
This is to add multiple domains behind the same IP, you also have the option to create multiple IPs (though I don't know how that works). Also, this is for Snow Leopard (and possibly Leopard), if you're running a slightly older version I think the only difference is that vhost.conf is included in httpd.conf.
Let me know if you have any luck!