How to horizontally center an unordered list of unknown width?

It is common to have a set of links in a footer represented in a list, such as:

<div id="footer">
    <ul>
        <li><a href="#">Home</a></li>
        <li><a href="#">About</a></li>
        <li><a href="#">Contact</a></li>
    </ul>
</div>

I want everything inside div#footer to be centered horizontally. If it was a paragraph, you would just easily say: p { text-align: center; }. Or if I knew the width of the <ul> I could just say #footer ul { width: 400px; margin: 0 auto; }.

But how do you center the unordered list items without setting a fixed width on the <ul>?

EDIT: clarification - the list items should be next to each other, not below.