I tried the Cmd-drag thing on some folders that no longer are where they were when I originally added them to "Places," saw the puffy cloud, was happy, and dragged some new folders to the "Places" sidebar menu. Everything was fine. Then I rebooted...
The old phantom "places" are back. The ones I had added--and wanted--are gone.
I have been stuck with these zombie folders in "Places" for many months. I tried getting rid of them in Leopard to no avail, and now the same miserable thing in Snow Leopard. Cmd-drag appears to work, but doesn't. Can anyone rid me of these haunts? Arrrrrrrrrgh!
UPDATE—SOLUTION: I just found out how to kill the beasts, so I thought I'd post it here for anyone who ran into the weird problem I had with Cmd-drag not working. A sure fix is to go to your user folder, then to Library/Preferences, and drag the file named com.apple.sidebarlists.plist to the trash. Then log out and log back in. Your sidebar will have been deloused, and restored to defaults.
If you have a lot of things in the sidebar that you want to have there (in addition to the problem things), do a selective screen grab (Cmd-Shift-4, then drag around what you want to grab) of a sidebar before you trash the .plist. That way, you can open that when you log back in, and use it to set the sidebar back up the way you want it.