This is one of those products where i go "why didnt i think of that before" This BMX seat from Eclat very cleverly combines a one piece plastic core which forms the seat and post. There is an alloy sleeve in the core to prevent it being crushed when tightening. Nice simple idea which saves a few grams for the weight junkies out there by removing all the bits and pieces to clamp the seat to the post....

http://www.eclatbmx.com/products/618-unify-combo-seat-unpadded