Apple's new "Put Back" feature in OS X 10.6 is convenient for items you have in the trash which you would like to restore to their original locations. Before Snow Leopard, you would have to manually locate and drag the file back to where it came from. Despite this convenience, there may be times when you select files to restore, and the "Put Back" option in the "File" menu is greyed out, and is missing from contextual menus.
When an item is placed in the trash, the system keeps track of its original location for the "Put Back" feature. If for some reason a file does not have this information associated with it, the system will not be able to use the feature. This can happen, for instance, if you uninstall or update a program and previous files are moved to the trash by the program. The Finder is the program that associates the original location information to a file when it is put in the trash, so only files moved to the trash through it will be able to be "Put Back".
Usually, you can select multiple items in the trash and use the "Put Back" feature; however, if one selected file does not have any original location information, then the "Put Back" feature will not be enabled. If this happens, you will have to use the "Put Back" feature on files one-by-one, which can be done easiest by using the "Put Back" keyboard shortcut (command-delete, which also moves selected items to the trash in the Finder).