On the control panel software i use each site in IIS handles www and non-www sites. I'm currently in the process of going thru all the sites, removing the non-www site handling and creating secondary sites with a 404 handler that redirects to the www version of the site. This is to avoid canoconical issues on google between www and non www. e.g. if you mix your links up and have some inbound links for www and some for non-www google treats them as seperate sites, one of them will get duplicate content penalties and the links will be split between the 2 sites. by using a 404 redirect on a second site to the first one(non-www -> www) using a 301 redirect I can transfer any pagerank/inbound link juice so it all counts for the one site (and you dont get duplication penalties. I'm doing this because im on IIS and i dont have access to a rewriter(yet).
Darren