Update 2011: Since Internet Explorer now always just display a vertical scrollbar, all that is needed to force a scrollbar in all other non-IE browsers is this:

html {
    overflow-y: scroll;
}

I worked on a site a few weeks ago where the user wanted visible scrollbars no matter how much space the content takes up on the page. Internet Explorer is happy to oblige, just put in a overflow: scroll; in your CSS and your good to go. However, Mozilla Firefox wouldn’t play ball. A bit of Googling revealed Mozillas own implementation:

overflow-y: scroll;

So to sum up, add the following CSS code to your site if you want the scrollbars visible at all times:

html {
     overflow: -moz-scrollbars-vertical;
     overflow: scroll;
}