At my blog (http://c133.org/blog/) I have a div#footer element with position: fixed specified in the CSS.
The correct rendering (as rendered by a WebKit nightly on the Mac, revision 30793) can be seen at http://c133.org/tmp/c133-macwebkit.png - while the rendering from the GTK build of WebKit can be seen at http://c133.org/tmp/c133-gtkwebkit.png and seems to be otherwise very good.
Created attachment 19549 [details]
GtkWebKit rendering showing lack of footer
This is a screenshot of the GtkLauncher app rendering the page; notice how the footer is not rendered.
Created attachment 19550 [details]
Mac WebKit rendering including footer
This is how the latest Safari nightly renders the page, properly including the footer.
Created attachment 19554 [details]
screenRect was returning a 0x0 sized rect seems to prevent such pages to render correctly.
Comment on attachment 19554 [details]
Landed in r30892.