I've upgraded by Xfce to 4.4 RC1 using the graphical installer. (Previously I had Xfce 4.4 Beta 1 on XUbuntu 6.06).
Everything seems to be working well, except that
* when I quit, I now have to enter my password to shutdown or restart
* when I quit, the hibernate and suspend options are missing
* when I am on battery power, the battery icon no longer shows (I think this was a Gnome applet
that also ran in Xfce)
* the compositor does not work
Since the upgrade installed in /usr/local rather than /usr, I had a message about the Trash service not starting properly, so I moved a lot of duplicate files from the old version in bin and lib out of the path. It did not fix the above issues, though.
A quick search on the web shows that some other people have similar issues, http://ubuntuforums.org/showthread.php?p=1506232
I forgot that I posted the original, sorry about that.