Thanks to these efforts we can use applications that are written in both these different toolkits in both desktops including their icons/theme engines, etc. Anyway, I don't know about you guys, but I've always been amazed by KDE's artwork!. I mean from the themes to the ability to actually control the environment, KDE is an excellent desktop.
With the introduction of Qt4 and KDE4... the new artwork (icons and the mouse pointers, etc) was called Oxygen and they look totally gorgeous as well. Sadly, we couldn't use them in Gnome (other than you run KDE apps in Gnome). But recently, someone from Gnome-look.org has created a Oxygen icon theme for Gnome 2.+ called Oxymentary.
But remember the icon theme is not a complete set, yet. For instance, currently it does not have all the replaceable icons for applications under Gnome desktop. If you have installed the elementary theme, then whenever the Oxymentary cannot find appropriate icons, it'll use the elementary icons instead.
If you want to give them a try, then please download from here. Then simply drag-n-drop them in "Themes" window or extract the content to /usr/share/icons/ and then choose them from the "Themes" (should be looking for the name "Oxymentary"). Enjoy!.