I have FSA MegaExpo Gossamer compact crankset... It developed the creaking problem about 2-3 weeks after I bought the bike. I took it in to have it checked out since I get free service within the year... the creaking stopped for about 30 min... now it's back full force.
My creaking problem is EXACTLY the same as yours. Have you isolated the problem? I haven't looked at my bike yet, but I am guessing it could be the crankarms... If you could post your findings, that would be much appreciated. Thanks.