PowerPoint 2000 says no:
If you want your presentation to include a series of slides that have a portrait orientation (taller than wide) but your presentation is set up for landscape orientation (wider than tall), you must first put the slides with the portrait orientation into a new presentation, create a link to the presentation, and then open that presentation during the slide show. If you want to create a link to an individual slide with a different orientation, open the presentation that includes the slide, and then move that slide alone into a new presentation.