When I began the installation of QuickTime 4, it wanted to shut down all active apps before proceding. I hit "Continue", and it proceed to send quit events to everything that was running.
However, NiftyTelnet is running, whenever sent a quit event, it first prompts the user to confirm that it should really close any active connections. It seems like the QT4 installer choked on this, and decided to freeze.
I started force-quitting apps until I got to NiftyTelnet. After Nifty closed, the installation completed normally.