All my colleagues with an iPad never have it with them. It's strictly a 'use at home' type affair for them, because most of them do a lot more with their laptop than the iPad affords. So they either carry their laptop, or their iPhone (which we all get from work).
There will always be different devices being better for different circumstances. And just as there will always be people who will prefer to have as many of those functions into one, there will also be a lot of people who don't need that, and who will be much better (and cheaper) off with a more specialised device. I personally carried my PSP around a lot for years and if I get an NGP, that will probably happen again. But I'm sure there will be others who think that's too much of a hassle. If I didn't have an iPhone from work, I don't think I would get one now. For the NGP, it depends on what it can do.
For me personally, if the NGP can do Android apps, then I think I would get a tiny 40 euro phone with a tiny bit of credit on it or a 2.99 euro subscription or something like that, and use the NGP for everything else. If it only supports Playstation Suite stuff, and that only gets a tiny fraction of the apps available for Android, I may still end up getting one of the cheaper Android devices. For now I still have the iPhone from work, but although I like it a lot, I'm also starting to get fed up with it - screen is too small to be comfortable enough to do the tonne of things you can do with it (it suffers from its own success there - I want something larger than this, but small enough to still fit into my pocket), and input is too limited for meaningful games beyond the Angry Birds type affair.
You have to assume that at some point the current market share for all phones and handhelds out there combined is the future marketshare for smart devices. That's a big market, and you'll find devices from small to large, solar powered, usable under water, with 3D screens or projectors built in, watch shaped, whatever.