You can use FireBug in FireFox. Right-click on the form and 'inspect element with firebug' open the css part and watch which properties define the position. Change them in the css files. Be sure to use the correct level in the hierarchy.
Hai Mike, like Chris said you can do this easily by inspecting the element in Firebug and create some custom styling in your theming. With some css skills it should be easy. Here is a start to add in your stylesheet:
The !important overrides the default position in the center of the page.