I really don't think of the feature in Bing as "voice to text" in the same way it is in the SMS app, but I suppose that's a semantics debate.
Big D5: I seem to remember Jerry Hildenbrand saying that Vlingo worked really well with his southern acccent, moreso than Google's built-in voice features.
And I'd say there's at least a couple of reasons MS didn't put the voice button on the keyboard. One is space -- the WP keyboard feels full as is, and if you've ever added another language you know just how cramped an additional button can make it. Secondly, they're focused on introducing features when they feel they're ready, not when they're ready as a public beta (ie. Google). Third, Microsoft, as best I know, doesn't have as much as a vested interest in capturing data from users to create a profile of them that can be sold to marketers. Pretty much everything Google does can be traced back to that.