http://www.target.com/Home-Pears-Cork-Placemat-Green/dp/B001GN6X6K/sr=1-10/qid=1238071649/ref=sr_1_10/192-6641846-6001156?ie=UTF8&frombrowse=0&pricerange=&index=target&field-browse=1038576&rank=price&rh=k%3Aplacemat%2Cp%5F36%3A%240-%2424&page=2

it is a corkboard placemat at target. the cork is good because it can be cut. things stick to it easy. i like it

How thick is it and how much weight will it support are important considerations.

I think that cork may be too soft and flexible for base - using glues on it may make it bend a little...that would destroy diorama.
I use cork on dioramas but never for a base.

i am using it for my base, i really hope that it wont warp. ugh. i really hope not

How does it behave when you try to move or rise the diorama? Is it stiff enough?

EDIT
Oh, by the way - I think that for 5-8$ you can buy nice picture frame which is harder than cork and makes a nice framed base for your dio.

i havent tried to yet. i only have grass mats glued down right now

if you are using it to build up your ground work you shouldn't have any problems, so long as you use a solid base. as habit, I always spray coat base and ground with enamel paint to seal and water proof, before appling plaster or other matterial. make sure all buildings and trees are secured to solid base or you might loose them in a 1/35 scale earthquake , it happened to me. you can get real strange base,s at the salvation army or the like and at yard sales. just look for picture frames in the size you like to build. also, old metal lamp stands work too and can be ornimental.