Everytime, no matter what browser I use, when I click "sign up", I get 503 Service Temporarily Unavailable
Maybe they do not like you
BTW I tried it and sure enough I got that error. Then when I refreshed the page I got the sign up page. Try to go to here:
https://mtgox.com/signupThen hit refresh. Worked for me. (on Chrome, IE did not work)