You can make the swf bg transp in html with the following:

1) go to file->publish settings->HTML
for "Window Mode" choose "Transparent Windowless"

this will add the correct param tag to the resulting HTML generated when you publish the movie (Shift-F12)

2) go to the html page in which you embedded the swf movie and add the following tag INSIDE the object tag:

<PARAM name="wmode" value="transparent">

Note: transparency only works with the IE browser, and does not work with Netscape....

Then just place a solid bg in the areas you want the bg to show.