Hello.

I want to use a single object of document A in another document B. How do I do that? Copy & Paste doesn't work.

use the import feature under the File menu item

Copy and paste only works between different windows of the same instance in inkscape other than the current SVN development versions. Open the second doc using the file>open menu command from the first documents window rather than with explorer or whatever and it should let you c/p between them.

Thank you, that did work. Just another note for others: make sure the layer you want to paste into is not locked.