mtgox.com does work for me, and then i get the green security bar aswell. So
www.mtgox.com probally directs you to a phishing website.
No, the certificate was signed only for mtgox.com (and not
www.mtgox.com), that's why you get that warning.
typing mtgox .com doesn't work either. First time I've seen the
correct website name might lead to a phishing site.