I wrote about this very topic for Small Business Resources and my answer is that YES it can happen a) if you have the right accessories and b) depending on what your needs are. You can get all your email and documents on a smartphone but if you need to do a lot of typing and/or reading a lot of documents a notebook computer is probably better for you.
Jamie Lendino of Smart Device Central writes Fast forward to 2006; what’s changed? The biggest difference between then and today is the rise of the smartphone, which is essentially a cell phone merged together with a PDA. While the standalone PDA certainly has its place, most people these days find that phones have all the PIM power they need. Plus, many smartphones now have QWERTY keyboards in lieu of quirky handwriting recognition. Credit that one to Research in Motion, whose Blackberry handhelds have sported full keyboards since the turn of the century.
No one really suggests that you can type up 20 page reports on a Treo. But many people take along laptops just so they can read their e-mail and check the Web. This is precisely the kind of thing that a smartphone is ideally suited for. When you’ve got one machine that weighs 5 pounds or more (with AC adapter), and another that weighs just ounces even with its accessories, the choice is obvious.
Whatever you decide, make sure you backup your data and that your computer experience is secure.