I use my laptop, and when i plug and hdmi screen, the panel is stuck in the screen "one", and the "configure panel" box does not propose to move it to the second screen. After restarting the panel, the configure-box proposes a choice between the 2 screens. (running xfce 4.6.2 from ubuntu karmic i386)
The positioning and multi screen code in the devel panel already supports all this. This is too complicated to fix in the 4.6 panel, so will close the bug when the devel panel lands in master.
Devel branch has been merged in master. A 4.7.0 release will follow soon. If you think this bug is not fixed? Feel free to reopen the bug.