I do think that mobile devices will become more and more useful and even necessary. I posted awhile back a post about the many things that a smartphone can do and it is a long list:
Son wants to waste money
I also find myself doing more and more online (although not necessarily using a mobile device) and am annoyed when I can't do some things online -- the restaurant without a menu and nutritional info online, the vendor where you can't pay online, etc.
I have an iPad and a smartphone (currently an Android phone, previously had an iphone) and use them all the time.
That said, I'm sitting here with my brand new Falcon Northwest Tiki computer and looking at one 26" monitor and another 24" monitor. I love my iPad for consuming media but for actual writing or doing things beyond that -- give me a desktop PC.
And, I play World of Warcraft which requires a real computer not an iPad. And I love the power of a gaming computer.
That said -- I often have to realize that I am not typical. I was talking to a co-worker recently who said that her PC at home is gathering dust as she uses her iPad almost exclusively. Many people primarily use computers for consumption of information and find the iPad enough alone. It isn't enough for me, but it probably is enough for most people.
The ideal device would be one that could serve all these functions. Could be a tablet like the ipad. Could have an effective keyboard for heavier duty use. Could run all the PC programs and hook up to dual large monitors. Some of that exists now, but not all of it in one package.