Scripts made in Adobe CS3 may not work in Adobe CS4 product. I tried an evaluation version of Photoshop CS4, and found out that my watermarking action 'crashed'.
The crash is triggered by the CS3 version of the 'Fit Image' script/command.
So much for backwards compatibility...
UPDATE: I've updated the watermarking action. This one works with CS4. For CS3 you need this one.
UPDATE 2: New CS4 Watermarking Actions