You can run gnome-theme-manager from the command line to set the style settings for the session, but they won't be remembered after a restart. Plus, doing this will start gnome-screensaver.
GTK Styles and Fonts comes with gtk-qt-engine (at least that's the package name on Fedora), but I found that if I use it, the OpenOffice toolbars get messed up.
It was all working fine for me in Fedora Core 4, but now in FC5 (on a different machine) I get the same problem as a550ee. No idea what I've done differently...
I use Plastik widgets in KDE, and even if it used Clearlooks widgets for the GTK apps I'd be happy.