I have a CanoScan 8400F and have it working perfectly under Lion. I'll try to explain what I did:
First you need drivers and software, go here:
http://usa.canon.com/cusa/support/consumer/scanners/canoscan_series
Select the Canoscan 4400F (YES 4400F) drivers and software, select Mac OSX 10.7 and download the following SOFTWARE (NOT Drivers):
CanoScan Toolbox 5.0 Opener (Mac OS X 10.6/10...
Next select the CanoScan 8400F drivers and software, select Mac OSX (NOT OSX 10.6, JUST OSX) and download the following DRIVER:
8400F Scanner Driver Ver. 10.2.4.1X (Mac OS X...
File Name: 8400fosx10241en.dmg
Next, still in the CanoScan 8400F drivers and software, select Mac OSX 10.6 and download the following SOFTWARE:
CanoScan Toolbox Ver. 4.9.3.4X (Mac OS X 10.2...
File Name: cstbosx4934ej4.dmg
You can open all of these images, but DO NOT INSTALL ANYTHING YET.
Turn off your scanner, unplug the power (yes the power) and unplug the USB cable from the scanner.
Install:
8400F Scanner Driver Ver. 10.2.4.1X (Mac OS X...
File Name: 8400fosx10241en.dmg
Next install:
CanoScan Toolbox Ver. 4.9.3.4X (Mac OS X 10.2...
File Name: cstbosx4934ej4.dmg
Restart your computer.
Plug the power and USB cable into the scanner and turn the scanner on.
Open Image Capture in your applications folder - The 8400F should show up, if not; with Image Capture still open, unplug and then replug the USB cable in (scanner still turned on).
With Image Capture open and the 8400F showing, run the last program you downloaded:
CanoScan Toolbox 5.0 Opener (Mac OS X 10.6/10...
You should be able to close Image Capture and open CanoScan Toolbox. Everything works, even the buttons on the scanner.
I have done this on a virgin install of Lion on an external drive, and it works there also.
Hope this works for you !!