Stéphane, one of our readers working with Photoshop CS3 announced a problem described on forums Apple and which has also affected him.
An incompatibility arises (at least with certain configuration) between Photoshop CS3 and Mac OS X 10.5.3. It causes the corruption of the saved files (at least .psd and .tiff) making them impossible to reopen.
It seems that this bug causes a corruption to the header of the file. While waiting for a solution, the only work around seems to downgrade to 10.5.2.
[Update] The corruption seems to occur only if one opens a file created under a version previous to 10.5.3 and then to save it. It will be then illegible.
On the other hand, a file created under 10.5.3 and then edited will not pose a problem.
[translation by crispin]