If you own an HP Photosmart C6300 series and upgraded to Ubuntu 12.10, you may have noticed that you are unable to set the scan resolution (DPI) in any of your programs. gscan2pdf has the scan resolution dropdown menu greyed out and you will be unable to get xsane to present you with a list of resolutions.
in the section called photosmart_c6300_series (it has to be this section, not the generic photosmart section).
For those who don't know how to do this, if you open up a Terminal window and type the following, you should, once you've entered your password at the prompt, get a visual editor that will make this easier:
sudo gedit /usr/share/hplip/data/models/models.dat
Hope that helps somebody!