Are you by any chance using an 'Intel' Mac?
If you are then it's going to be because the Scanner drivers will still be PPC and running in emulation under Rosetta. CS3 however has been updated and runs Intel 'native', and so is unable to load in the driver/plugin for the scanner.
There is a work around to force Photoshop CS3 to run under Rosetta (get info on the Photoshop app, and click the 'Open using Rosetta' box), but this will cause Photoshop to run slower.
This really bugs me, Why cant Canon get there act together and release a Intel plugin for there scanners (the NEW scanners do come with an universal driver but they don't work with there older scanners)