I posted this same issue in the Xfce support forums
this morning. So far, no replies.
This morning, using Synaptic, I finally upgraded my main Wheezy Xfce 4.6 computer to Xfce 4.8. There were 225 upgrades, plus 33 new things to be installed.
It has been about 2 weeks since I last fully upgraded that system, so I'm sure that not all of today's upgrades were directly related to Xfce 4.8.
I waited to upgrade as long as I did because Wheezy's been getting a steady stream of Xfce 4.8 upgrades for the past several days -- I wanted to wait until that stream stopped, to reduce the chances that I would run into an upgrade problem that was on the verge of being fixed.
The only issue I had was that, in the middle of the configuration step (for lib6, I think), it asked me to either stop or restart xscreensaver before continuing (so that I wouldn't get locked out of the current session), so I completely stopped xscreensaver.
No other issues, error messages or prompts, which surprised me, because I was expecting it to ask me whether I wanted to install a default panel or upgrade my existing Xfce 4.6 panel.
When the upgrade finished, my desktop background and panel looked exactly the same as they had before the upgrade. I had (wrongly?) expected that at least the new panel would look different.
So I clicked on the "Quit" icon at the right end of the (only, bottom) panel, to shut down and restart my system, thinking that then I would see the new Xfce 4.8 panel.
But, when I clicked on the "Quit" icon, instead of seeing the familiar Quit/Restart/Logoff dialog box, an error popped up:
Failed to quit the panel.
No running instance of xfce4-panel was found.
A Google search told me that a bug was reported about that same issue affecting Ubuntu Natty several months ago -- but I haven't seen any reports about it or published fixes for it re: Wheezy.
So I'm using my other PC (Xfce 4.6) to post this message.
Now I'm afraid to use my Xfce 4.8 PC and -- since I don't know what's wrong with it -- I don't want to mess around with it and possibly make it worse.
At this point, I don't even want to try using the Xfce menu to log off or restart it, because -- from what I've read about other people's Xfce 4.8 panel problems in the past several months -- I don't know if the panel will even be there afterward.
This is probably caused by something minor -- but to me, it's something major, because I don't know what else it might be affecting, so it keeps me from trusting or using that PC until I know specifically what to do to troubleshoot and fix it.