I expect the harware in the 3G is just not upto the job - or at least not to do it to the satisfaction of Apple.
Don't forget that the current apps people typically cite in the discussions on this topic do all the voice recognition server side.
The google app for example just records your voice as a data file, sends it off to google to do the recog. then fires back an answer to what it thought you said.
The 3rd party voice memo apps (reqall, QuickVoice) do the same trick.