Christian,
Not natively no, but you can work around it by putting the portrait
(vertical) slides in a separate presentation and linking to them in your
normal presentation.
See here for some explanation as to the why and how you can work around it:
http://www.rdpslides.com/pptfaq/FAQ00042.htm
Luc
"Christian Jorgensen" <Christian (e-mail address removed)>
schreef in bericht