Several recent KDE 4 snapshots show Plasma widgets embedded into the panel. However, it was never really mentioned how exactly these widgets can be added to the panel. If the user tries to drag the widgets from the desktop to the panel that won’t work: instead, the widget will disappear behind the panel.
The right way to add widgets to the panel is:
Drag the widget directly from the Plasma widget browser to the panel and drop it there.
That just works. However, some widgets are not yet ready for a problem-free panel integration, so the panel might get screwed up – but a simple right click on the problematic widget gives you the option to remove it again.