Most of the many bugs I run into with OS X involve the combination of managed users (less than standard user privileges), fast user switching, and devices "owned" by a single user (try switching users while an iPod is associated with a user).
I wonder if this bug falls into that category. I'd updated QuickTime and iTunes and restarted. Everything seemed well, but Aperture wouldn't launch fully. It displayed the splash screen and hung -- as though it was waiting for something. No SPOD, no spinning wheel, nothing.
I tried a few times and decided to restart. My iMac then hung on shut down, with the "spinning radial lines" effect. I left to do a few things, and 20 minutes later it was still "spining". Then I noticed the iPod shuffle light was blinking yellow. I pulled the shuffle from its cradle and the system shut down immediately. On reboot, with the Shuffle removed, Aperture opened normally.
Presumably on startup Aperture is polling disks, and the Shuffle was in an odd half-mounted state (OS X is prone to dismountus interruptus).
Something worth remembering ...