Voice is the primary medium and use for cellphones; no surprise there. It is the primary medium for communication purposes. While voice has great potential as a medium for delivering information, voice is also more intrusive, and linear in nature, thus slower, than the visual representation of the same information. Opposite to this, voice as means of application command (or navigation) input is great.
Voice as a medium is important though for accessibility (for the impaired), an area we are not spending enough time addressing (within our own products).
P.S. I’ve updated my piece On iterations, mobile, texting, web, and local applications, to reflect voice as an important medium for information delivery to consider when designing applications.