I had this problem rarely on Leopard and since installing Snow Leopard a few days ago its been more common. Random applications (usually iTunes, System Preferences, Disk Utility, and SystemUIServer) freeze and show up in ps with a U next to their name and stick around as zombies after they're killed. It seems like it's related to the CD drive (opening Disk Utility or the Disc Burning tab in System Profiler) or SATA (opening Serial-ATA in System Profiler also sets it off)

Laptop is a HP HDX 16t running 10.6.2
Intel Core 2 Duo
Chipset is ICH9, I think
NVidia GeForce 9600M GT 512MB (using NVinject)
Intel IWL5100abgn - not working, using wi-fi ExpressCard and i386 kernel for Atheros drivers
Realtek RTL8169 ethernet
Also using VoodooHDA for audio, VoodooPower, VoodooBattery, and some other kexts that were installed automatically by the Hazard 10.6.2 dvd