The attached diff modifies the 2.0.1 version of CRM/Core/Payment/PayPalImpl.php to prevent requiring users to enter their name/address info twice, once on our Contribution form, and then again once we send them off to Paypal Standard. Should work for any Contribution/Membership/other form that includes standard contact info fields. Is agnostic about which address is used (Primary, Billing, etc).
This propagates the variables I cared about (first, last, address1, city, state, zip, email). If more variables are desired, the doc on Paypal's variable names is here:
*NOTE* the attached diff was generated against my locally SVN'd cop of CiviCRM so the revision number in the patch won't match the official repository.