Might be an Intel plugin or maintenance issue. I could not duplicate this behavior on a PPC [2.3 GHz DP G5, 1 GB RAM, OS 10.4.11, Photoshop CS version 8.0 (8.0x119)] with 5 consecutive saves of the same file after minor retouching between saves. Have you tried opening it using another admin account? How about running some maintenance program like Onyx or DiskWarrior?
Otherwise, the Multiprocessor update you are referring to only works and is recommended for multiprocessor computers. Fortunately, it is a plugin stored in Adobe Photoshop CS>Plug-Ins>Extensions folder. I have it (PPC version) and there are no problems. You might like to drag yours to desktop and see if it changes the save functionality. If the action is restored and there no other adverse effects just trash it. Adobe actually recommends to save a copy of the old plugin before applying the update. You could also try reapplying the update. Please post back with your progress.
Alex
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Alex
2.66 GHz 17" MacBook Pro, 4 GB RAM, OS 10.5.7, Office 2008, TimeWarner Cable