Until Apple add the function to hide the keypad like on the iPad there is no way of minimising the keyboard in this scenario.
One way around it would be to read the message received and then hop over to notes and type the reply, and just keep switching between notes and messages to review and type some more then just select all the text in notes and cut then paste into the message once you have finished with it.
Saying that though it is a cumbersome way around things and as the post above says, use mail. It's a lot better