I'm heading for Amsterdam this Sunday (June 28, 2009). I will activate my Eurail pass immediately on arrival and will travel to Paris. I assumed that it would be impossible to make a reservation for this train trip because the pass would not be activated yet, BUT -- is it possible for me to make a reservation (and pay ONLY the supplemental fee) now, even though I have not yet activated the pass? Many thanks, anyone who knows.
Reservations are independent of passes or other tickets. You should be able to make a reservation at any time, but of course you have to show a valid pass along with your reservation when on the train and the conductor comes around and asks to see your ticket.
I agree with Tyler. Remember that the pass you have purchased covers you for the distance travelled, and proof of that is only required while you are travelling on the train. Reservations just book the seat and do not require any proof of having a ticket (or pass) at the time of the reservation.