Yes, I'm using lxdecc and I can add, rename etc. my virtual desktops.
I reinstall obconf in Synaptic. It doesn't solve the "problem".
Maybe it have something to to with the lxpanel? I noted this evening that this is the same with the lxpanel in Openbox Educational (for my kids).
If I go in the panel parameters, in the "Panel applets" tab ("Applets du tableau de bord" in french) I notice that i can modify the menu,taskbar etc. but I can't modify the virtual desktop (pager plugin), just move it up and down.
Sorry for my english, I hope I explained it clearly!
Thanks for your help Neal!