NO, not really.  But ...

If you are using PowerPoint to project the presentation onto a screen (or
monitor), than try to picture the slide edges as the screen edges.  You will
not be running out onto the stage to rotate the screen from landscape to
portrait and back (well I hope not anyway).  Just insert whatever onto the
slide image and size it to fit within the screens borders.  So even if the
picture within the slide is taller than it is wide, you will still want to
project it using a landscape page set-up, since this is the screen's
orientation.

If you are using PowerPoint to create printouts ("Publisher Lite" if you
will).  Than simply create two presentations: one for all  the landscape
pages and one for all the portrait ones.  You will then need to manually
collate the two outputs.  If you do not want to collate, use Word,
Publisher, or a print-based software instead of PowerPoint.

If you are using PowerPoint to design a web site, I'd strongly recommend
against it.  There are far better tools for this that allow for mixed page
orientations.

As Bill says unless youre turning the screen around ....

But if you must

http://www.technologytrish.co.uk/ppttips_landscape_portrait.html