As Debian will not ship XFCE 4.10 in unstable until after Wheezy was released and this is very unlikely to happen before spring 2013, (460 RC-Bugs to fix) we decided that we want XFCE 4.10 now for our users. Parts of it are suffering bitrot in experimental, others needed to be packaged still.
We are happy to let our XFCE users update their installations now (thanks to agaida and towo). All you need to do is, as root:
echo deb http://ftp.spline.de/pub/siduction/xfcenext unstable main \ > /etc/apt/sources.list.d/xfcenext.list
That will put a file called xfcenext.list into your sources.list.d folder and write the appropriate line to it. Then you can install or update an existing XFCE with:
apt-get update && apt-get install xfce4
Have fun with your shiny new XFCE.