Some interesting responses on this and all of them are true in their own way.
I got very excited by the iPad and created my own app to show my work, showed it to a couple of excellent photographers who then wanted to buy it, so I released it. I called it Foliobook. So I've had cause to reflect on these viewpoints over the last year as I've 'eaten my own dogfood' and used it to show my work, or to supplement my print book.
1) Nothing beats a print portfolio... for high resolution, impact, emotional contact with fine work.
2) The use of an iPad to display a portfolio is a different beast. I take my iPad on shoots, if for no other reason than having TPE with me at all times (The Photographers Ephemeris - fantastic app). The iPad is more available and casual than a print book, if I meet someone, I have Foliobook on the iPad with me and whip it out. Also it shows timelapse movies far more effectively than a sheet of any kind of funky rag paper. It's also easier to change the portfolio on the fly, maybe between meetings. My own app allows turning off sections and subsections so its quick to remove and add back parts of the content. But the iPad is electronic, this creates a different relationship with the photograph that may not be appropriate in some situations or with particular content. iPad 3 with double the resolution may change this. Sometimes the iPad is a definite negative when the person viewing the work is unfamiliar with the iPad... in this case they can focus totally on how amazing the iPad is, and will forget to address your work properly because they are busy fiddling with the device or talking about it.
3) The Photo's app that comes with the iPad is great so long as you don't want to determine the order your clients/prospects decide to view your photos in and if you also don't mind missing an opportunity to drill your brand into their brain. Not to do that seems a waste of an opportunity. In its favour is that the iPad photos app is written by Apple so the quality level is very high and sets a level of expectation that most of the third party portfolio apps struggle to reach. The number of sales of photo organisation and portfolio apps says that the Photos app doesn't meet everyones needs and of course its not meant to address all needs. And often not those of a professional photographer.