I'm not using the On Desktop function, actually.
When my system starts up from Hibernation, the Events window is always in the upper-right quadrant of the desktop. (Not the specific part of the corner that I set it at.) Sometimes I can move it, other times it behaves like I'm painting in Photoshop. (No grabbing possible.)
Today I found another instance of all programs appearing under the calendar, instead of over it. Even the Rainlendar context menus appeared under the calendar, instead of over it. Odd.
If the development of Rainlendar will take some time, I'll investigate using the Vista sidebar and calendar gadget. It's not as useful, nor does it have all my dates configured.