I never knew/realised this, but it’s very useful to know:

The background of a Flash movie can be set to transparent. This allows the background color or image of the HTML page that contains the Flash movie to show through and allows the layering of Flash content with DHTML content.

Very useful when your Flash is on a layer in HTML that could be covered by something else when you’re scrolling the page.

Wmode not set:

Wmode not set

Wmode set to “transparent”:

wmode set to transparent

Source: Adobe TechNote

PS: Never mind the awesome (ahem) colours, this was just a quick test. 😉