We can consider adding that to checkout pages we control, such as our Free Checkout page and card-based direct payment checkout pages for Authorize.Net, PayPal Payments Advanced and PayPal Payments Pro, so we'll put that request on the wishlist.
For other checkout methods such as PayPal Payments Standard and Google Checkout, we don't have any control over the information requested on their checkout pages; if the buyer uses their existing account with those services, the email address listed in their account profile would be used and already verified as valid when they set up the account.
