Suggestions

• To-button + number field should be smaller, or it should be on a different screen once you press send(AKA more space for content)
• Send+backspace should move to the bottom, IMO
• Display name instead of number
• Tap and hold backspace button -> clear the whole text area
• Tap and hold a number button -> Jump straight to the number(AKA insert the number in the SMS)

That's all I can think of now. Oh and either use a database you create or use the existing Conversations database(that would be more desirable). If you use the Conversations database, maybe you could make it more like the Conversations app, so you can read SMS'es as well.