It's a template modification of the navbar template, not a hack.
Go to your admincp and open your stylemanager. Select "Edit Templates" under your default style. Scroll down and find "Navigation and Breadcrumb templates" double click that (in IE). Select navbar.
Look for this code
Code:
<!-- / nav buttons bar -->
<br />
Beneath it add the code you want for your site logo. You can use <center> tags or put it in a standalone table and center its contents.