Place an item in the cart, then continue through the checkout process til
you get to the address entry screen.  Uncheck "Use my billing address" and
then enter all but one piece of required information in the shipping
address section.  Now, click Continue Checkout and will get returned to the
address entry screen and told there are errors (because of the missing
field.)  But, looking down the page, you will see that "Use my billing
address" is checked again.  This should not happen.  The state of that
checkbox should be retained.