Even aside from the obvious considerations of smaller screens and clumsier textentry, smartphones and other mobile devices come with different user expectations.
Rather than adopting the tried, true and traditionally inefficient hunt-and-peck alphabet, Valve is introducing Daisywheel -- a familiar looking textentry setup designed specifically for analog gamepads.
In the original Kindle, you had a dedicated search button which would bring up a textentry field, then transport you to a page to select how you wanted to search, then to your results.