A workaround, as suggested by Simon and the Gitorious support, is simply to use the SSH protocol instead of the Git protocol, even for read-only access to repositories. This requires to have a Gitorious account setup, which most of us should have already.
You can edit your .git/config file to achieve that by changing the line
url = git://gitorious.org/webkit/webkit.git
url = firstname.lastname@example.org:webkit/webkit.git
Do the same for the QtWebKit team repository:
url = email@example.com:+qtwebkit-developers/webkit/qtwebkit.git
Gitorious are reporting having difficulties to handle all the traffic they currently have, I guess this may affect the stability of the daemon serving the
git://protocol, who knows.