I haven't seen it discussed, but transcription is one of the most important features to Google and it is a large reason why they are willing to offer Google Voice for free. Why you ask? Training. Google voice's free transcription is a huge voice to text training database.
I have been using it since before it was Google Voice (e.g. grandcentral) and this was an important reason for Google to acquire it.
Google gets a LOT of value from every voice mail that comes in, is transcribed, and then is rated by users as to how useful it is.
Yes, it is good already, but not nearly perfect and they are working on it for one reason - voice search. And voice search is an up-coming Google killer-app that Bing/MSFT and Yahoo have no answer to.
(Neither does Apple, yet.)