i don't think that's possible ?
but i did find something close to that:
source:swftools.com

swftools Wrote:A great tool for creating interactive wallpapers with dynamic content, animations. SWF Desktop 1.2 adds support for Flash MX 2004 and Mac OS X.

• SWF Desktop you can develop wallpaper that looks good AND delivers dynamic content, animation, interactivity and more.
• SWF Desktop supports Windows 95 through XP and Mac OS X.
• Because SWF Desktop takes advantage of Flash MX, your wallpaper can download and display new content every day with as little or as much effort from the user as you design into your swf.
• Take advantage of XML support in Flash and put up-to-the-minute information on the desktop.
• With SWF Desktop you can integrate links right into the desktop making your online auction, expert resource, blog, user group, or favorite internet rag one click away.
• SWF Desktop wallpaper can stretch to fit any screen size.
• SWF Desktop enables you to create a one-click wallpaper installer in two easy steps
• End users download and run the installer and your swf becomes their current desktop. No complicated install instructions to follow. No need to open any control panels.

dl link : here


EDIT : i think this might work  " swf desktop"
problem is.. i can't find a download

if you know any html you could embed the swf into a html file and set that as the desktop background. I've made  a few background slideshows in the past  just  as an experiment and the worked pretty well.

Games like a Grrl Wrote:Active desktop?  I need to do some html coding then, eh?  K, shall start studying.

i use dreamweaver for that :he:

Games like a Grrl Wrote:they're all about $30, and I'm just curious to try it out, not sure if I want to commit the money to it.

who wants to commit money on these things anyway... :he:

This should help a little.

Create the HTML with this in it.

<object width="550" height="400">
<param name="movie" value="somefilename.swf">
<embed src="somefilename.swf" width="550" height="400">
</embed>
</object>

Course you need all the other http mumbo jumbo.  Of course the file name and size will have to change to suite your file.

:)

Hope this helps.

@Wolf, try setting  a realy small gif, like an animated bullet point as your  background and set it to tile. click ok and tell me how long before you get a headache.