create a div and use its z-index to have its content over the table's, both widths should match, hehe.
<table style="width:600px; cellpadding="0" cellspacing="0">
<div style="text-align:center; position:absolute; z-index:2; width:600px;">text over cells</div>
or superimpose tables
<table style="width:600px; height: 400px; position:absolute; top:0px; left:0px; " cellpadding="0" cellspacing="0">
then the rest of the table...
then the superimposed table
<table style="width:600px; height: 400px; position:absolute; top:0px; left:0px; text-align:center;" cellpadding="0" cellspacing="0">
<tr><td>your text</td></tr></table>