Xubuntu in that sense is one of the well respected Linux distro which uses the Ubuntu "engine". Now they've released an Alpha version of this upcoming release called the 11.04. It can now be downloaded if you're interested in experiencing the latest software versions, etc. According to Xubuntu alpha release note...
|A clean and a simple looking theme... looks good to me :D|
*. xfce4-notifyd, xfce4-appfinder, xfwm4, ristretto, xfce4-dev-tools and xfce4-volumed are updated to latest versions.
*. No PPC or ps3 images are available in the Alpha releases.
*. Still uses the GTK2+ to keep the stability with the xfce4.8
*. Although not very cleaner but they seems to be going to introduce a new theme as well as pointed out by Andrew in his recent blog post.
If you want to try it out then - Click here to visit the official downloading page.