This sounds exactly like what I was experiencing last week. Creaking that I could have sworn was coming from the headset...and at one point I was convinced that it was the cables making the noise as well. Well, last weekend I put my bike on the Cycleops fluid 2 we just bought to check it out (and this was after my LBS spent some time checking out the creaking noise and we thought it was fixed) and lo-and-behold the noise was still there. So I had my wife come out and see if she could pinpoint where the noise was. Turns out that I was certain that the noise was coming from the front of the bike in the headset area and it turns out I was completely wrong. It was the seat post the whole time. Took it out, cleaned the old grease off and reapplied new grease and it hasn't made a peep since.