I haven't set it up yet, but it is supposed to have virtual hosting fixed. Check out this
thread on arsdigita.com about 3.1 with ACS. It appears that there are custom parts of AOLserver3.0+AD6 that are not in 3.1. I am not sure if the affects OpenACS or just ACS. There is a changed fancy .adp parser, and two modules that ACS 4.0 uses that are not in 3.1 yet.
