Premier Trains
 

If you've got a Eurail pass can you ride the fast trains (ES, ICE, TGV, Thalys, etc.) for the price of the reservation or do you have to pay for a ticket (with a discount?) aswell as the reservation fee?

Thanks =)

For the price of the reservation and or supplement:
with pass
ES about $15 fee - get drink and pastry in first class
ICE- no additional fees, reservations not required
TGV - 3 euro reservation fee
Thalys - about $12 in 2nd class, about $28-30 in first - in first get a meal with drinks
so basically the train fare is paid for by the pass - mandatory reservation fees not or supplements which often get a little something on board

Sounds like you've been reading the RailEurope brochure that talks about 'discounts' on premier trains. Generally they are not really discounts but mandatory reservation and supplements as i list in above post.
But if you have a railpass that only covers part of the journey on say a Thalys train (Paris-Brussels-amsterdam or Paris-Brussels-Cologne) - a pass covering only one of the countries you pass thru then you'd get a discount - perhaps paying 50% of normal fare.
You talk about ES trains - i assume you meant Eurostar Italia but if you meant Eurostar Paris/Brussels-London then the pass gets you a passholder discounted fare - currently $75 thru RailEurope.
I recommend the free European Planning & Rail Guide from BETS (800-441-2387) as this magazine answers all your questions about the premier train fees and is a great primer on European rail travel for novices - rail maps, travel times, fares, itineraries, etc. I originally read about it in Lonely Planet guides as one of their suggested rail resources and found it extremely informative.

So I would just show my pass and then just pay the reservation or supplement fee and then I can get on and go? :)

Yes but you have to go to the ticket window with your pass and buy the reservation/supplement before boarding.

Does that apply to the Eurostar chunnel train between London and Paris too?

In theory it does... but passholder fares ($75 each way) though available on every day of the week, including more expensive weekends, are sold in a very limitet allotment on some trains and can be hard to get at the ticket window and you may have to pay much more for a regular ticket. So though you could possible show up and get one it's unlikely many days and last i priced them in London they were charging $95 for a passholder fare, which though varies as to the dollar vs pound exchange rate. So i recommend booking in advance thru BETS (800-441-2387) or other RailEurope agent to be guaranteed a passholder fare. Note that Mon-thu there are $90 round trips that have nothing to do with a passholder and these would be cheaper if going round trip - but these are even harder to book as the day of the train approaches so also should be booked far in advance. Can book 9 months in advance. If you have any questions on Eurostar call BETS - they'll take time to answer without obligation.