I had the same thing, my seat seemed to creak when i put pressure on my pedals, (uphill, accelerating etc). I took it to my LBS and the guy there who is ace tried my bike out in the car park and said it could be my seat post, he took it out and oiled it re-positioned it and it WORKED. I'm very particular about creaks, squeaks and clicks and my version of hell was a clicky pedal all the way to skeggy from Nottingham. It sorted it and now i ride creak free.
WHUPP