Googling turned this up: http://data.agaric.com/localhost-from-virtualbox-xp-install-ubuntu
It suggests using IP: http://10.0.2.2, and it worked for me.
So, I edited the hosts file, C:\windows\system32\drivers\etc\hosts, and added this entry:
10.0.2.2 outer
If you’re testing on IE8, remember to put http:// in the address bar. Just putting the ip directly will not work.
For example:
http://10.0.2.2:3000/