I read the article about nulls (http://www.sqlservercentral.com/articles/articlelink.asp?articleid=2161) and thought the same as the guy in this post below
http://www.sqljunkies.com/Forums/ShowPost.aspx?PostID=9211
If you read the article without reading the forum replies then you'd think that nulls were better for performance, but his test scripts had flaws, like bad comparisons and no indexes! Read the thread and you'd come out thinking that it was slightly better to have NOT NULLS!
Just shows that you should always read the posts about an article...