Ok, I see what's going on.
It's because you have EVERYTHING absolutely positioned. To fix the problem you're going to have to redo your markup.
I'm not sure why you have absolutely positioned. Maybe you have a valid reason, though I can't think of what that might be. I'm guessing it's because you want the related vids div to the right and perhaps don't know about CSS floats.
Anyway, I cleaned up your code a bit, added 1 line of CSS to the #relvids id, and added one extra div that surrounds the related vids and the vid details divs.
The main problem was two-fold:
The issue about the comment div not expanding is because you had declared it's height. Now, it will with more content if it were filled, but the footer div was absolute positioned under the comment div (at that stated height), so even as the comment div expanded the footer would be underneat the text. Now, I've removed the height. If you enter more comments, the div will expand, and the footer will move down with it.
I suspect that your code could be cleaned up more but it's hard to know with just seeing a wireframe. Why do you have the headings in their own divs for example? Also, why do you have heights and widths specified for these? You're also declaring a lot of classes (headers, dtlist, dtheading, etc..) that aren't in y our CSS. Not sure if you included your entire CSS in your post though, so this might exist elsewhere.
Anyway, here is fixed up version.
<html>
<head>
<style>
#container {
width:760px;
margin: 0 auto;
}
#title {
width:400px;
height:25px;
}
#rightad {
width:285px;
height:288px;
}
#flashplayer {
width:450px;
height:400px;
}
#vdetails {
width:450px;
height:235px;
border: 1px solid #CCCCCC;
}
#comments {
width:450px;
z-index:7;
border: 1px solid #CCCCCC;
}
#relvids {
width:285px;
border: 1px solid #CCCCCC;
/* Edit - Add */
float:right;
}
#detailshd {
width:70px;
height:20px;
}
#commentshd {
width:190px;
height:20px;
}
#details {
width:435px;
height:185px;
}
#relvidhd {
width:151px;
height:28px;
}
#footer {
width:760px;
height:110px;
border:1px solid #ccc;
border-width:1px 0;
background-color: #E1E1E1;
}
#copyright {
position:relative;
left:280px;
top:125px;
width:200px;
height:20px;
z-index:1;
</style>
</head>
<body>
<div id="container">
<div id="title">
<h1><<NAME>></h1>
</div>
<div id="rightad"><a href="http://www.linkfame.com/peternorthdvd/go.php?pr=8&su=1&si=100&pa=index&ar=&ad=177340&campaign=1384" target="_blank"></a></div>
<div id="flashplayer"><<FALSHPLAYER>></div>
<!-- Container that holds "relvids" & "vdetails"-->
<div id='container'>
<div id="relvids">
<div class="heading" id="relvidhd">Related Videos: </div>
<br />
<br />
<<RELATEDLIST>>
</div>
<div id="vdetails">
<div class="heading" id="detailshd">Details:</div>
<div id="details">
<table width="100%" cellpadding="5" cellspacing="0">
<tr>
<td colspan="2" ><span class="dtheading">- From:</span> <span class="mediumlink"><a href="<<FROMMOVIELINK>>" target="_blank"><<FROMMOVIE>></a></span>
<div class="mediumlink"></div></td>
</tr>
<tr>
<td colspan="2" class="dtlist"><span class="dtheading">- Length:</span> <span class="mediumtext"><<VIDLENGTH>></span></td>
</tr>
<tr>
<td colspan="2" class="dtlist"><span class="dtheading">- Views:</span> <span class="mediumtext"><<VIEWCOUNT>></span></td>
</tr>
<tr>
<td colspan="2" class="dtlist"><span class="dtheading">- Starring:</span> <span class="mediumtext"><<STARRING>></span></td>
</tr>
<tr>
<td colspan="2" class="dtlist"><span class="dtheading">- Date Added:</span> <span class="mediumtext"><<DATE>></span></td>
</tr>
<tr>
<td colspan="2" class="dtlist"><img src="graphics/LightLinePixel.gif" width="435" height="1" /></td>
</tr>
<tr>
<td class="smalltext">URL:
<label></label></td>
<td><input name="URL" type="text" id="URL" value="<<URL>>" size="60" /></td>
</tr>
</table>
</div>
</div>
</div>
<div id="comments">
<div class="heading" id="commentshd">Add your comments: </div>
<br />
<br />
<<COMMENTS>><br />
<br />
<label></label>
</div>
<div id="footer">
<div align="center">Footer Navigation and Copyright </div>
</div>
</div>
</body>
</html>