Here's a cheat sheet for all the WebKit repositories.
SVN: http://svn.webkit.org/repository/webkit. This is always the latest and greatest WebKit code since SVN is the version control used for WebKit code. To checkout trunk, you would do 'svn co http://svn.webkit.org/repository/webkit/trunk'.
Git mirror at webkit.org: git://git.webkit.org/WebKit.git. This git repository is a mirror of the svn repository. If you work with the Qt/WebKit port, you should _not_ choose this (for reasons explained below).
Git mirror at Gitorious.org: firstname.lastname@example.org:webkit/webkit.git. This git repository is a mirror of the svn repository. The difference is that the author/committer names in the commits are "cleaned" up. For example, in the webkit.org's repo it might say 'email@example.com@268f45cc-cd09-0410-ab3c-d52691b4dbfc'. In gitorious' repo, this is cleaned up as 'firstname.lastname@example.org. As a result of this cleanup, the repository sha1's of gitorious and webkit.org _don't_ match. This mirror is run by Tor Arne. If it is not up to date for some reason, you now know whom to poke.
qtwebkit at gitorious.org: git://gitorious.org/+qtwebkit-developers/webkit/qtwebkit.git. This is the QtWebKit repository. The releases are essentially branches created from the gitorious.org mirror with patches/hot fixes applied. These patches may not have yet landed upstream. And the commits might be in a different order. Since, qtwebkit is branch off gitorious.org, QtWebKit developers would find working with the gitorious.org easier. If you fetch gitorious.org mirror often, you will have lesser objects to fetch to keep qtwebkit uptodate (as opposed to webkit.org git mirror which has nothing in common with qtwebkit).
My usual git-svn workflow:
git fetch webkit-gitorious (get git-svn commits)
git update-ref refs/remotes/trunk webkit-gitorious/master (change the trunk)
git svn rebase (makes git-svn update the svn<->sha1 mapping)