As per bug #150493:
This is "true" transparency using XComposite, not blending with the background or a screenshot.
It's fairly buggy, but that's caused by xcompmgr and Metacity's ignorance of Compositing (I think):
- When xcompmgr is running and you create a new note, it's not transparent although xprop says _NET_WM_WINDOW_OPACITY is set to a correct value. Restarting xcompmgr makes new notes transparent
- Text of notes can't be changed
- Notes can't be resized or moved
The "Delete" and "Sticky" buttons work fine though. Really strange.
Metacity really needs a built-in compositing manager, like XFWM's.