I just set up 6 low traffic virtual hosts with each having an OpenACS instance with one IP under AOLServer 4 on FreeBSD 5.1 thanks to Vinod's docs here:
http://www.panoptic.com/wiki/aolserver/98
As people have mentioned each OpenACS instance loads A LOT of tcl, but it works. It takes a significant amount of time to restart. I had to add a swap file (http://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/adding-swap-space.html), but otherwise things went fine. For non-heavy development, low traffic sites, it seems ok.
