I wanted to point this out for any SEO's that are concerned between an individual WordPress with www and a WordPress Multi-site non-www url. In the past, my company has optimized top ranking sites on static html sites and WordPress sites. The biggest horrific derailing of search engine positioning we've noticed is WordPress' non-www url on multi-sites. If we have inbound links that have point to www. and change/accidentally switch it to non-www the site will bounce around the SERPS. We initially noticed this last year (2009) when one of our sites dropped off the front page off Google search results. Switching it back to www. brought it back within a couple weeks.
Today we discovered another site bouncing around the SERPS the past couple of weeks. The culprit: non-www URL - somehow during the transition from 2.7 to 3.0.1 the www. in the site URL was removed. This is a individual wp install.
It's unfortunate we can't use Multi Site Domains for these already well-established ranking websites. I don't understand why Google would treat www. and non-www sites differently. I really wish www. and non-www was an option for Mu installs. :(