CentOS 5.x Linux with MongoDB 2.0.1 (tried main and legacy-static)
MongoDB is running:
root 31664 1.5 1.4 81848 11148 ? Sl 18:40 0:
I don't see this having an accepted answer yet, so I'll just add my 2 cents.
I had the exact same issue just now. After a while I realized I've locked localhost out in my iptables rules. So, check your firewall.