I had to do three things to get my old but nice Canon CanoScan N650U flatbed scanner to work with Adobe Photoshop CS3: 1) I had to download the latest driver for this model, which included a Photoshop plugin. 2) I had to manually move the CanoScan Photoshop plugin into /Applications/Adobe Photoshop CS3/Plug-ins. 3) I had to run Photoshop under Rosetta, the PowerPC emulation layer in Mac OS X for Intel Macs, because Canon's drivers do not yet run directly on the Intel hardware.
To run Photoshop under Rosetta, find /Applications/Adobe Photoshop CS3/Adobe Photoshop CS3, right-click (or control-click) on it, select Get Info. In the panel that opens, check the "Open using Rosetta" checkbox. From now on, Photoshop will open under the (slower) Rosetta PowerPC emulator. The loading screen (with the logo and the status messages) will say "Running under Rosetta".
