Having OmniWeb add "www" on every site will cause all kinds of problems.
Consider going to, say, mail.yahoo.com only to find that it's now
www.mail.yahoo.com. (okay, bad example, Yahoo did the Right Thing and made
www.mail.yahoo.com redirect to mail.yahoo.com)
Domain admins SHOULD redirect or otherwise manage things so
www.domain.com is the same as domain.com.
But, for now, just add the www. prefix if it's necessary.