No, I only have dns configured for 
http://www.domain.tld, not for admin.hostname.domain.tld.  I would assume that 
http://www.domain.tld would go to the virtual directory I added to ispcp and not to the admin page.  If anything, the admin page would not resolve at all??  I am using a single external IP and would like to have admin.hostname resolve to ispcp and www. resolve to the virtual site.  
Adding a host entry for admin.hostname.domain.tld pointing to my internal IP allows me to resolve to the control panel properly.  I am not real concerned with external access to the panel now but I would like to get to the virtual site.