How can I prevent a click on a '#' link from jumping to top of page?

In jQuery, when you handle the click event, return false to stop the link from responding the usual way prevent the default action, which is to visit the href attribute, from taking place (per PoweRoy's comment and Erik's answer):

$('a.someclass').click(function(e)
{
    // Special stuff to do when this link is clicked...

    // Cancel the default action
    e.preventDefault();
});