Figured out the problem.
First off, to troubleshoot the problem, click Window->show Log
Second, the actual problem was the installer was hanging during: ifpkinstallelement
A bit of googling and I randomly found that it could be caused by iPhone Simulator running simultaneously.
I close iPhone Simulator and the installer instantly resumed. (It failed shortly after that, but a restart and re-running the installer without iPhone Simulator running solved the problem.)
So, do not run the installer with iPhone Simulator running.