I've never endorsed a product before, but I have to tell you that for the past couple of weeks I have loved, loved, loved PDF to Keynote. It does just what it says -- it takes a PDF presentation and makes a Keynote presentation out of it. It's free software.
For whatever reason, Keynote doesn't import multi-page PDFs. I've been making Beamer presentations for one of my classes, and create PDF output. So now I can translate these into Keynote presentations to use the presenting tools there, with the Beamer-generated slides and outlines. It's a simple tool, and it does one thing well. Pretty cool.
I should mention, although I like Keynote for presentations, it's never available at conferences. I always show my conference presentations as PDFs. Powerpoint always messes up something -- graphics don't import, colors change, fonts are the wrong size so text falls of the edge of the slide. This happens whether you're going from Keynote to Powerpoint or from one version of Powerpoint to another. Those computers always have Acrobat installed, and you can show a PDF of a presentation full screen. Nothing will change; it will look just like it did where you created it.
