I haven't confirmed if this trick can be achieved on the default or other home launchers, but just now, I was able to overlap widgets together. Could be a bug with LauncherPro.

I can actually press in between the top layer of widgets to access the underlying Facebook widget.

It's easy:

1) When you press-and-hold on a empty area to add a widget, a 4-item menu pops up.
2) Select "widget" -- but immediately switch home screens before the list of widgets appear.
3) Select the widget you want.