Ok. This is what you have to do

1) Login to WordPress Mu admin
2) On left side menu “Site Admin” -> “Blogs” and click on your blog
3) Change site url from http://yourblog.com to http://www.yourblog.com
4) Scroll down and press “Update options”

Now all non-www requests will be redirect to www version.

(The procedure is similar in regular WordPress where you need to change the “Blog Address URL” in admin -> Settings -> General)

Of course to work this you need to have apache mod_rewrite installed and activated on your server.

Keeping only one url version is search engine friendly. Say you allow your site to be accessed by all these urls. www.yourblog.com, yourblog.com and yourblog.com/index.php. All these point to a same page. This can be lead to pagerank splits, Backlinks split and lastly you’re risking getting filtered for duplicate content by the new filters at Google.

For these reasons it is better you keep either www or non-www version activated but not both.