Which ulimits (stack, data, rss, nofiles, etc.) does the AIX kernel enforce? It appears that the rss (memory) ulimit is not actually enforced, for either the soft or hard value. If that is the case, what is the point of having unenforced ulimits?
Having ulimits that are not enforced helps make scripts portable. They won't crash when they try to set a ulimit.
Did you have a look at the ibm developer works? They have some excellent resources. http://tiny.cc/hqRpD