AFAICT, more features need to be added to the ecommerce package before a paypal gateway would meet the requirements of the business rules on page 17 of the Paypal Integration Guide[1]. Some of the requirements have been added to the bugtracker as suggestions/"to do"s (for other reasons).
Alternatively, one could add a parameter to ecommerce, that when set, passes the shopping-cart data to paypal for processing[2]. You would need to create a thank-you page to go with it. This is the quickest implementation method, but has many drawbacks.
1. https://www.paypal.com/en_US/pdf/PP_WebsitePaymentsPro_IntegrationGuide.pdf
2. http://www.paypal.com/cgi-bin/webscr?cmd=_pdn_howto_checkout_outside
