The margin of auto on the sides of the div leave it up to the browser to decide where it goes. There is nothing telling the browser that the div should be centered in the body, or left or right aligned. So it's up to the browser. If you add a directive to the body, your problem will be solved.
<html>
<head>
<title>Welcome</title>
<style>
body { text-align: center;}
#pageContainer {width:300px; margin:0px auto;
text-align:center; border:thin 1px solid;}
#toLogo{border:none; }
</style>
</head>
<body>
<div id="pageContainer">
<a href="http://portal.thesit.com" id="toSite">
<img src="LOGO_DNNsmall.png" id="toLogo">
</a>
</div>
</body>
</html>
I added a 1px border to the div so that you could see what was happening more clearly.
You're leaving it up to the browser because it's in quirks mode. To remove quirks mode, add a doctype definition to the top, like so:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
<head>
<title>Welcome</title>
<style>
#pageContainer {width:300px; margin:0px auto;
text-align:center; border:thin 1px solid;}
#toLogo{border:none; }
</style>
</head>
<body>
<div id="pageContainer">
<a href="http://portal.thesit.com" id="toSite">
<img src="LOGO_DNNsmall.png" id="toLogo">
</a>
</div>
</body>
</html>
Now you'll be able to see your 300 px div center on the page.