For what it's worth, the git version of org, with those settings,
doesn't leave an extra line. If you're willing to consider running the
development version of org you'll get that and a whole bunch of other
new features. If not, you might look at the function
`org-add-planning-info' as it exists in the git codebase, and see how it
differs from yours (particularly when the symbol 'closed is passed in as
the third argument).
http://orgmode.org/w/?p=org-mode.git;a=blob;f=lisp/org.el;h=05f5375f4e47bdfe6de55d6aa8f2ec7130dedaca;hb=HEAD#l12230
Also, there's a orgmode mailing list!
Good luck,
Eric
--
GNU Emacs 24.1.50.2 (i686-pc-linux-gnu, GTK+ Version 2.24.10)
of 2012-05-19 on pellet