Apparently it hangs effectively forever. I let it sit for an hour as my longest trial. I ran in debug mode and the log file shows this:
014/03/28 12:34:37 ui: [0;32m local-jenkins: Installing Chef... [0m
2014/03/28 12:34:37 /usr/local/bin/packer-builder-virtualbox-iso: 2014/03/28 12:34:37 opening new ssh session
2014/03/28 12:34:37 ui: [1;32m==> local-jenkins: Provisioning with chef-solo [0m
2014/03/28 12:34:37 /usr/local/bin/packer-builder-virtualbox-iso: 2014/03/28 12:34:37 [DEBUG] Background SSH connection checker is testing
2014/03/28 12:34:37 ui: [0;32m local-jenkins: [sudo] password for vagrant: % Total % Received % Xferd Average Speed Time Time Time Current [0m
2014/03/28 12:34:37 ui: [0;32m local-jenkins: Dload Upload Total Spent Left Speed [0m
2014/03/28 12:34:38 ui: [0;32m local-jenkins: 100 14401 100 14401 0 0 35568 0 --:--:-- --:--:-- --:--:-- 35646 [0m
2014/03/28 12:34:41 /usr/local/bin/packer-builder-virtualbox-iso: 2014/03/28 12:34:41 [DEBUG] Background SSH connection checker is testing
2014/03/28 12:34:42 /usr/local/bin/packer-builder-virtualbox-iso: 2014/03/28 12:34:42 [DEBUG] Background SSH connection checker is testing
2014/03/28 12:34:47 /usr/local/bin/packer-builder-virtualbox-iso: 2014/03/28 12:34:47 [DEBUG] Background SSH connection checker is testing
2014/03/28 12:34:52 /usr/local/bin/packer-builder-virtualbox-iso: 2014/03/28 12:34:52 [DEBUG] Background SSH connection checker is testing
2014/03/28 12:34:57 /usr/local/bin/packer-builder-virtualbox-iso: 2014/03/28 12:34:57 [DEBUG] Background SSH connection checker is testing
It looks like the "Background SSH connection checker" keeps trying every 5 seconds, and never passes that point.