Your example link is broken and your subdomain is inaccessible with a HTTP 403 error.
Try not to use Transitional XHTML, use CSS and Strict XHTML instead. So your table could be like this:
<style type="text/css">
.var{width: 600px}
</style>
<table class='var'>
<tr><td colspan='2' style="text-align:center">RE This is one cool forum.</td></tr>
<tr>
<td style="width:100px;text-align:left">Dylan<br />Posts: 10</td>
<td style="text-align:left;vertical-align:top'><p style="text-align:left'>Me to.</p></td>
</tr>
</table>
Ok I could have did more to improve that code but I kept the way you had it and CSS-ized the code. You should try setting table header cells and the absolute (or relative width of those cells). Based on that your table is 600px wide, I'd recommend:
<table class='var'>
<tbody>
<tr><th style="width:100px">Posted by...</th><th style="width:500px">Message</th></tr>
<tr><td colspan='2' style="text-align:center">RE This is one cool forum.</td></tr>
<tr>
<td style="text-align:left">Dylan<br />Posts: 10</td>
<td style="text-align:left;vertical-align:top'><p style="text-align:left'>Me to.</p></td>
</tr>
</tbody>
</table>
Also, the <th> contents will automatically be bolded and centered. If you set the absolute width of those cells, the rest of the columns have that width. But you only have 2 columns, the 'RE' one is a spanned cell, so I don't know exactly what you mean by the next cell having an offset because there is no cell beside it. But try my suggestions and see if it works.