Yes.
It would be very easy if the IP address that somebody was posting from was static. The web administrator for the site can just do a lookup on the IP address, see what ISP was allocated those IP blocks, and then contact the sys admin for the ISP and see what user the IP was assigned too.
From a corporate level, if some for of firewalling was in place, a sys admin could track to see what sites users were hitting.
It would be very difficult to get the identity of an individual poster without the help/consent of the IPS/sys admin of IP block though.