I think this usually happens on machines running 64 bit Ubuntu. An updated package hasn't been released yet, but the necessary patch is in git master for xfce4-session, so you could compile from there. Otherwise, probably best to use another desktop environment if you are doing critical work on your machine.
Extra note for other readers: (If you are using the xfce 4.10 ppa on 12.04, the fixed package has been released; I'm not sure whether there has been an official update yet for 13.04)
Compilation from git:
(For build-dep to work, you need to go to software sources and enable source code by checking the box. Then run sudo apt-get update and any subsequent build-dep commands should work.)
sudo apt-get install build-essential git xfce4-dev-tools libxfce4ui-1-dev libxfce4util-dev checkinstall
sudo apt-get build-dep xfce4-session
Now, gain the source from git and build (I compared the default options from the package source and they are no significant differences, apart from debug level)
git clone git://git.xfce.org/xfce/xfce4-session
cd xfce4-session
./autogen.sh --prefix=/usr --sysconfdir=/etc
make
sudo apt-get remove --purge xfce4-session
sudo checkinstall --pkgname=xfce4-session --pkgversion=4.10.1 --provides=xfce4-session
Now logout and login to activate the new session.
As this is development code, there might be other issues. So if you have any problems preventing you login at the lightdm screen with this new xfce4-session, press ctrl-alt-f1 and login at the console and remove this package and reinstall the normal repository version. Then return to the lightdm screen with alt+f7.
This package may be overwritten (depending on the versions) when the official fix comes out, but that will be fine.
You can of course remove this version and reinstall the package version, although this new version should solve most of the issues.