I have suddenly encountered the same issue with both self-mounting and .dmg images. Double-clicking in the finder will open Disk Copy and it will verify the image and state that the image is mounted, but it never appears anywhere on the desktop or on any of my volumes. Searching through Sherlock (even for "invisible" files) can't locate it either. If I try to trash the .smi or .dmg file, I'm told that I can't because "the file is in use..."
Apparently the system thinks the thing is mounted, but I don't have a clue where it is.
I just installed the 10.1 update to 5G64, and this is the first time I've encountered this issue. I'm not sure if it's a bug or what, but I've never had this problem with earlier versions of 10...