Application with invisible window

[weasel <weasel beyondnormal org>]

I would like to create an application that in effect has an invisible 
client area in the window that can be drawn to.  I can accomplish this 
with transparency and mask, but then each pixel is either on/off, what I 
want is alpha translucency.

For example:
Lets say I have a number of png images with alpha channels.  I would 
like to create a window that does not display a background. (what ever 
is behind it would show through.)  I would then like to draw these png 
to the window so that the areas of the images with alpha levels would 
also let what ever is behind them show through as appropriate for the 
amount of alpha.

Is there some way of setting the background of the a window equal to a 
pixbuf but so that it in effect replaces the background with the pixbuf 
instead of just drawing it over the default background?

Thanks,
Nick