Hi,
Paypal need the address of the customer, so in your checkout you must have Login - Address - Payment steps, but set login option to "no login" and registration to "no registration".
By this way you still have the address step required by paypal, and you just have to make a translation override for the "register" button, change it to "Use this address" (for example).
Here the customer will just have to enter his billing address, and go to paypal.
That's the fastest way actually, paypal express plugin is not yet developed.