I don't have a Leopard machine to test on anymore, but nothing in recent versions of the code has changed that would affect Spotlight Items, and the last time I ran it on Leopard it was still working just fine.
Just to clarify, you are dragging items onto the Tag Folder icon, not into the window that opens to show the results, correct? The window that opens is just a standard Smart Folder, and therefore cannot receive files.
Can you drag events, messages, and contacts from a Finder search window to the desktop?
Open one of the Tag Folders that you are having trouble with, and please make sure that its search criteria is set to include Spotlight Items. (If required, the Tag Folders user guide has instructions on how to do that.)
Finally, do you have write permissions on the files you are trying to tag? It is hard to imagine that you wouldn't, but your situation does seem very odd.
Jon