AC was broken early on in the server's life. It was changed around Velious release and had a noticeable impact on tanking end-game content. No clue if it's been re-broken over the last year, though.
IIRC at the time of fix Rogean had mentioned there is a softcap at some value - no hardcap.
edit:
http://www.project1999.com/forums/sh...0&postcount=42
http://www.project1999.com/forums/sh...2&postcount=57