Have you ever concluded about this?
Now I've met one, very strange, don't know why.
My code:
<td valign="top">
<input type="text" />
<iframe scrolling="no" height="21px" frameborder="0" width="62px" marginheight="0" marginwidth="0" src="Server/SecCode.php">
</iframe>
</td>
But it turns out to be that the "input" is valigned to the bottom,while "iframe" to the top.
I experienced something similiar, when I added the css code
<td style=" vertical-align: top;" >
manually within the td tag it corrected the problem for me.