All the attempts I’ve seen to try and hack the browser file upload input tag have been quite bad. They mostly involve trying to put some other element over the browser created button, catching clicks on that element and then forwarding them to the original button. The mileage for this method varies with the browser used and also requires creating your specific image to cover the button, which gets tiresome if you want to change languages or colours.
Enter upload-at-click. A brilliant piece of work that works with every browser (even including IE6 !). Take a look at it and you’ll see why I’m impressed.