Currently, I use FlashVars within the HTML Flash Object tag to pass the variable, and in the SWF file I set up a variable to pass to the SWF Parameters for the video clip source. The source field is left blank in the SWF videoclip, but I have used ActionScript to set the source, and I call the video object "mymovie".
In the HTML file, I use FlashVars in the OBJECT tag, as follows:
<param name="FlashVars" value="mymoviesource=http://mysite.com/mymovie.flv">
In the SWF file, I use: