Came to say the same thing. Even better is just leaving out the background-image/text-indent trick altogether, and using the image's alt tag:

<button><img src="/img/sign_up_button.png" alt="Sign Up" /></button>

The versatility of the <button> element is really remarkable, and it works fine in all browsers. I highly recommend anyone currently using <input type="submit"> to change to <button type="submit">