Update: After considering and actually trying just about every suggestion made in this thread, we finally located the culprit of the creaking. It was the bearings in the headset. I had this same issue years ago with my road bike. I had already suggested that to the LBS. He said he didn't think that was the problem. He went out of town during the holidays and the mechanic completely disassembled the bike and checked everything. The bottom set of bearings felt rough. He ordered the part and started putting the bike back together, lubing/greasing everything as he torqued it all to specs.

When the bearings came in today, he put them it and called me to say he thought the bike was fixed. A test ride later by me confirmed his suspicion. Needless to say, the bike rode like a brand new one. I could've sworn the noise was in the crank area, but apparently the noise was resonating down the downtube.