This is what I see in Console System Log when opening XDCAM Transfer pointed to a folder.

Sep 2 11:44:55 craig-seemans-mac-pro XDCAM Transfer[11241]: XDCAM Transfer 2.10.0 (Build 565) Mac OS X Version 10.6 (Build 10A432)
Sep 2 11:44:55 craig-seemans-mac-pro XDCAM Transfer[11241]: Loaded component MXFImport.component from SharedSupport
Sep 2 11:44:55 craig-seemans-mac-pro XDCAM Transfer[11241]: Loaded component EXImport.component from SharedSupport
Sep 2 11:44:55 craig-seemans-mac-pro XDCAM Transfer[11241]: Loaded component SMPTE331MAudioDecompressor.component from SharedSupport
Sep 2 11:44:56 craig-seemans-mac-pro [0x0-0x17f17f].com.sony.bprl.xdcamtransfer[11241]: objc[11241]: Class QCResource is implemented in both /System/Library/Frameworks/Quartz.framework/Versions/A/Frameworks/QuartzComposer.framework/Versions/A/QuartzComposer and /Library/Graphics/Quartz Composer Patches/ImageDownloaderBlockingPatch-10.5.plugin/Contents/MacOS/ImageDownloaderBlockingPatch-10.5. One of the two will be used. Which one is undefined.
Sep 2 11:44:56 craig-seemans-mac-pro [0x0-0x17f17f].com.sony.bprl.xdcamtransfer[11241]: objc[11241]: Class QCCGLContext is implemented in both /System/Library/Frameworks/Quartz.framework/Versions/A/Frameworks/QuartzComposer.framework/Versions/A/QuartzComposer and /Library/Graphics/Quartz Composer Patches/ImageDownloaderBlockingPatch-10.5.plugin/Contents/MacOS/ImageDownloaderBlockingPatch-10.5. One of the two will be used. Which one is undefined.
Sep 2 11:44:56 craig-seemans-mac-pro [0x0-0x17f17f].com.sony.bprl.xdcamtransfer[11241]: objc[11241]: Class QCImage is implemented in both /System/Library/Frameworks/Quartz.framework/Versions/A/Frameworks/QuartzComposer.framework/Versions/A/QuartzComposer and /Library/Graphics/Quartz Composer Patches/ImageDownloaderBlockingPatch-10.5.plugin/Contents/MacOS/ImageDownloaderBlockingPatch-10.5. One of the two will be used. Which one is undefined.

Perhaps if we can compare this to one with a system without the thumbnail issue it may provide a clue.

Might be totally unrelated but I see this being repeated endlessly in the log

Sep 2 11:53:32 craig-seemans-mac-pro com.apple.launchd.peruser.501[171] (com.epson.epw.agent[11352]): posix_spawn("/Library/Printers/EPSON/InkjetPrinter/EPW/IJEPWAgent.app/Contents/MacOS/IJEPWAgent", ...): No such file or directory
Sep 2 11:53:32 craig-seemans-mac-pro com.apple.launchd.peruser.501[171] (com.epson.epw.agent[11352]): Exited with exit code: 1
Sep 2 11:53:32 craig-seemans-mac-pro com.apple.launchd.peruser.501[171] (com.epson.epw.agent): Throttling respawn: Will start in 10 seconds

Let us know if any of the system do/don't or have ever had an Epson Printer attached to it.

I'm also thinking this might be a key

Sep 2 11:59:51 craig-seemans-mac-pro [0x0-0x186186].com.sony.bprl.xdcamtransfer[11417]: objc[11417]: Class QCImage is implemented in both /System/Library/Frameworks/Quartz.framework/Versions/A/Frameworks/QuartzComposer.framework/Versions/A/QuartzComposer and /Library/Graphics/Quartz Composer Patches/ImageDownloaderBlockingPatch-10.5.plugin/Contents/MacOS/ImageDownloaderBlockingPatch-10.5. One of the two will be used. Which one is undefined.