On my commuter, I carry it on the rack pretty much CdCf. I don't have a rack on my other bikes (out of no need for them, vanity not an issue ). I just take the commuter any time I think I'll need to lock a bike up.

On my old hybrid commuter, I didn't have a rack and was able to carry the lock by attaching the mounting bracket to the seat tube, and mounting the lock backwards. The straight bar of the U-lock was parallel to the seat tube, and the U-part would rest against the seat stay (on the non-drivetrain side). Might not work with all frames, but it was a clean solution on that bike.