I recommend using tables with <td valign="middle">
(as inkedmn mentioned, it works in all browsers), but if you're wrapping with a link, here's how to do it (works in ugly old browsers too, like Opera 9):
<a href="/" style="display: block; vertical-align: middle;">
<img src="/logo.png" style="vertical-align: middle;"/>
<span style="vertical-align: middle;">Sample text</span>
</a>