I've had this numerous times. Unfortunately it happens after prolonged periods of uptime. It's an old bug that stems back to 2004 and has been a problem for owners of real Macs. Basically it's a cache problem with the launch services database that from time to time gets messed up due to too many processes. The culprit that causes this to happen is old non-Leopard proof applications running from within the system. The best thing to do is hunt down and remove any old apps to avoid this from happening. Also download this small app to rebuild the database. Make sure you kill the finder while rebuilding. Alternatively you could also use Onxy to clean out and rebuild the caches.

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MacBook Pro 15" | Mac OS X 10.6 • 2 GHz Intel Core Duo • 2 GB DDR2-667 • ATI Radeon Mobility X1600 128 MB • 100 GB Serial-ATA
Hackintosh | Mac OS X 10.5.6 • 3 GHz AMD Athlon 64 X2 6000+ • MSI K9N AM2 • 2 GB DDR2-667 • NVIDIA GeForce 8800 GT 512 MB • ALC888 Audio • 300 GB Serial-ATA
iPhone 3G • 8 GB