seatpost/saddle problem

Thought I'd try to use some of your experience in solving a problem I've got on my new bike set (just bought Kona Kula Supreme 18" 2008 frame):
I've got Thomson's zero layback seatpost and I've "eaten up" all my rail's space, and I need to get my saddle about 1 cm farther away (backwards) from the handlebar.(Using WTB Silverado saddle).

The reason I can't replace the stem is because it wont solve the problem I'm trying to overcome: my knees are a bit too forward when I set the crank arm vertical to the ground.

Should I be looking for a saddle with longer rails (if thery're, which ones ? XC racing) or do I have to replace the seat post to the laid back version ?

You answered your own question right at the end. You need a seatpost that'll put your saddle further back.

Thompson posts are great but their setback post looks rather odd.

If you find your saddle really comfy I wouldn't go changing it for something else.

Zero setback seat posts aren't really meant for most bikes. As you have found out, most bikes are designed with the assumption that the saddle will sit on a clamp that is behind the seatpost itself. Just get one like that, and you will be fine.