Currently there is no API which allows you to access the user's SMS. Though there is a way to launch the Messaging application with the "Body" and "To" filed programmatically filled in.
The Microsoft.Phone.Tasks provide you with various classes which allow your application to launch various inbuilt application in windows phone such as contacts application, sms application, email application, camera application, search application etc.
The SMSComposeTask class is what you need to launch the messaging application.
