<td id="centercolumn">
<{if $xoops_showcblock == 1}>
<table cellspacing="0">
<{if !$xoops_isuser}>
<tr>
<td id="centerTopcolumn" colspan="2">
<{php}>include("uploads/topblock_guest.html");
<{/php}>
</td>
</tr>
<tr>
<{/if}>
<{if $xoops_isuser}>
<tr>
<td id="centerTopcolumn" colspan="2">
<{php}>include("uploads/topblock_user.html");
<{/php}>
</td>
</tr>
<tr>
<{/if}>
<tr>
*** and then normal call to centerLcolumn, centerRcolumn and centerCcolumn. The ending </if> is at the end of the centercolumn section ***
Then I put the 2 html files in my uploads folder. This way there is a different first block to display depending whether visiter has logged in or not. Also, if you don't want the block to display, you can have a blank html with the same name, rather than having to recode your theme.html file.
BTW, my homepage is set to None, but I've tested this with it set to News and Forums and it still works. Hope this helps.