I'm building a WordPress site with swf backgrounds embedded using swfobject. I'm not sure about flv's, but the swf background was very doable-- the trickiest part was just a stylesheet and embed code issue to make sure it works on all browsers.
What I have that seems to be working well...
Initialize the swfobject in the header:
<script type="text/javascript" src="<?php echo(get_bloginfo('template_directory')); ?>/swfobject.js"></script>
<script type="text/javascript">
swfobject.registerObject ("flashContent","8");
</script>
Embed the flash object in the footer (with transparent wmode... I'm also detecting the WP pages to change the swf names):
<object id="flashContent" classid="clsid:D27CDB6E-AE6D-11cf-96B8-444553540000" width="2000px" height="900px">
<param name="movie" value="<?php echo(get_bloginfo('template_directory')); ?>/swfs/<?php
// can chech which page this is to change swf
?>.swf" />
<param name="wmode" value="transparent" />
<param name="bgcolor" value="#464646" />
<!--[if !IE]>-->
<object type="application/x-shockwave-flash" data="<?php // can chech which page this is to change swf
?>.swf" width="2000px" height="900px">
<param name="wmode" value="transparent" />
<param name="bgcolor" value="#464646" />
<!--<![endif]-->
<p>Please update your Flash Player</p>
<!--[if !IE]>-->
</object>
<!--<![endif]-->
</object>
Use CSS to position and fit swf dimensions (though I think you could do this and the embed code with percentages if you want to scale with the page):
#flashContent {
z-index: 2;
position: absolute;
width: 2000px;
height: 900px;
top: 0;
left: 0;
}
You can take a peek at the work in progress here.
Good luck!