I got a Canon Pixma MP640 connected via WiFi and decided to try some scanning.
Paintbrush 'From scanner or camera' was greyed out, along with Paint.net's Aquire option.
Picasa detected the scanner and launched the drivers 'ScanGear' but imported photos were all blue !
The solution was to install and use 'MP Navigator EX' which I had missed due to Canon only listing it under Vista.
Navigator also does OCR and was easy to use, except for the buttons looking greyed out when they are not.