Ubuntu should be accessible to everybody, including the visually impaired. The current state of the gnome-accessibility-themes package is not acceptable.
You will see that some menu entries and applications are fairly complete, while others only partially so. This image shows openoffice, which is the worst offender not using any high contrast icons at all, behind gedit which is complete. There is a bug filed against OpenOffice on this here.
The Ubuntu artwork team should help better this situation. Install the gnome-accessibility-themes package and get to work!
Jakub Steiners original high contrast theme with svgs :http://jimmac.musichall.cz/stuff/high-contrast.tar.bz2. Jakub recommends a 48x48 target size.
Also by Jakub Steiner - a small howto on designing high contrast icons. Might be a good base for some style guidelines: http://jimmac.musichall.cz/doc/high-contrast/html/index.xhtml
In his icon guide Jakub recomends the icons be tested by bluring them. This is an example a 15px Gassian blur was applied to an image from the icon development page:
And here the image was made gray scale and then blured: