Hmmm, 'very timely and interesting thread; I too have an slk compact crankset on a Merlin XL like eliu and I have an annoying minor creak that is not cyclical in nature (so I know it's not something lame like the drive arm hitting the FD cage, or some other physical contact). It does seem to happen on either ring and so far I have tried the following:
1) lubed and retightened the 5 alloy c-ring bolts. Did not overtorque
2) removed the non-drive arm and lubed the threads on the M? crankbolt and retightened the arm and bolt. Again didn't overtorque
3) Went for a 20something mile ride after 1&2 above, and no change, creak still there. It does seem somewhat sensitive to pedal pressure so I can fool myself for a little while by backing off the pedal pressure, but what fun is that?...
Though I haven't taken everything off the BB and measured like eliu has, I'm pretty sure that the shell width is within reason of 68mm, and the faces are parallel. I have a brand new SRAM 9-speed FW and a relatively new Shimano 9 spd chain (<100 mi when mated w/ the crankset from hell). The rear wheel discussion is a possibility as I have 10 YO Mavic Cosmics with the funky (IMO) cones, so maybe I should take the rear wheel off and back the cone off and see if I can juuust get the rear wheel to be snug w/ the QR tightened and no slop in the hub.. Alternatively I could try throwing another rear wheel on the bike and see if the creak goes away. It's just pretty annoying because the other bike that the test wheel would be coming from currently has a Gossamer compact on it, with the same chain (model, not physical chain), and Shimano 9-speed FW, and it's like buttah, so it's a bit disappointing for an SLK/MegaExo setup to be, uhm, lacking.
There are a couple more things I could still try, but, thanks, everyone, for chipping in with your helpful suggestions, I appreciate the thought and tenacity you guys put into this. I'll silence this thing yet!