In Kubuntu 11.04 (under VirtualBox), I had the default panel at display bottom, and set to auto-hide. Moving the mouse to the bottom of the display caused the panel to show.

In 13.04, this no longer works. It works if the panel is at display top or display left, not if it is at display bottom or display right. With the default panel at display bottom (or display right), if I move the mouse to the bottom (right), I see a dull blue line; if I move the mouse a few pixels up (left) the line becomes thicker and brighter, but the panel never appears. I can neither see nor configure the panel.

This may be related to how KDE sees the display in VirtualBox. In Kubuntu 11.04, the display is configurable in System Settings, and shows the display resolution; in 13.04, the display is named "VBOX0" and is not configurable (although clicking the information icon reports the correct display size).

I'm using VirtualBox 4.2.12 and KDE 4.10.2, Kubuntu 13.04.

Unfortunately Virtualbox (and other virtualisation software) has been known to cause unusual problems in these areas, as the edges of a VM window aren't actually the edges of the screen - so the host operating system could be interfering at the vital cross over point, etc.

Have you tried running it in full screen mode?