I'm running Nagios 3.0 on Ubuntu 8.04. I found that my status map was broken after install. ( It didn't show). I found this to because I wasn't paying attention when I was installing the GD-library which is a prerequistie to installing Nagios. Are you sure everything is ok with your GD-library?
( To fix I reinstalled GD. Then I reinstalled Nagios). Let me know if this worked for you.