From the Google Mobile Blog:
Using your voice to search for businesses is super useful in situations when you can't type, when the name of the business is long, or when you're not sure how to spell it. In other situations -- when you're in a library or a rock concert, for example -- typing makes more sense. Keeping that in mind, we designed this feature to allow you to choose whether to speak or type. Get it now on your BlackBerry Pearl by visiting http://www.google.com/gmm.
Built on the same speech recognition engine used for Goog411, Google Maps with voice search is very much like the Tellme service for Blackberry or Live Search with voice, as well as applications from V-Enable, Vlingo/Yahoo!, Nuance and others.
Google had been the last of the major providers without true "voice search," putting Goog411 aside. Now all the major Internet brands have or are integrating voice in mobile (save AOL).
It will ultimately drive more search and usage frequency. Recall that ChaCha reported 40 queries per month among many of its users and up to 150 for its heaviest users. This is about voice and its ease of use.