FLV is Flash Video, as used on Youtube and Google vids and just about everywhere these days. FLV files don't play themselves however. They need a player... here's one: http://flowplayer.org/

You embed it all like this:

<object type="application/x-shockwave-flash" data="FlowPlayer.swf"
    width="320" height="263" id="FlowPlayer">
  <param name="allowScriptAccess" value="sameDomain"/>
  <param name="movie" value="FlowPlayer.swf"/>
  <param name="quality" value="high"/>
  <param name="scale" value="noScale"/>
  <param name="wmode" value="transparent"/>
  <param name="flashvars" value="videoFile=your.flv&autoPlay=true"/>
  <param name="flashvars" value="videoFile=your.flv"/>
</object>

 

FCK can insert the player as an object, but to make it play your FLV file, you need to upload that as a file and then adjust the parameters of the flash object to suit. What FCK would need is an interface for setting the parameters of the player.

Jack... I'd have a look on the FCK forums and see if there's some discussion about it. I imagine they'd be looking into extending the FCK Flash Properties dialogue to suit. http://www.fckeditor.net/forums/

There's also Bizmodules' very nice Ultra Video Gallery module if you're planning on hosting a lot of videos. The module can convert videos to FLV either on the server or offline with a separate app.