Forgot to mention this above, but zmove has done some cool work theming his cart and checkout pages to include a walk-through of the checkout steps at the top of the page. You can view this by adding this product to your cart and checking out his pages. He's done some great work there, and he might be a good resource for any other Ubercart themers.

This sort of feature might be a good candidate for consideration in a default store theme.