yeah sucks! really wish there was a way to cancel sending the message. like the "send" button turns into a "cancel" button during sending. that would be ideal. hopefully apple will address it and help users like me out.
If you are jailbroke, install SMS Helper from Cydia. You can configure it's settings to enable send confirmation, which gives you a pop-up after you hit send that allows you to either confirm the send, or cancel.