If your keyboard comes up when you are going to reply and it has a microphone key symbol in it, just tap the microphe key and talk when it prompts you. Then just be sure to enunciate clearly.
yes...i'm able to do that. i'm referring to the app of voice search from google. the lasy at the store and several you tube videos show you just being able to hit it and say "send text to...." then you say your message and it goes.
Go to the Market and download Google Voice Search. That will update the software on your device. To activate, just press and hold the search button for a second, and then say your command.