more in support of using 'subsite':
A google search for 'subsite' returns over 500,000 hits. Certainly many are related to ACS, but many are not, too.
Regarding the usability of 'subsite', Jakob Nielsen defined[1] subsite in 1996 consistent to how it is used at OpenACS.org.
1. http://www.useit.com/alertbox/9609.html
