The documentation is somewhat scarce, but my search has not turned up anything but SMSComposeTask for sending text messages from an app.
Will this be the only way to send SMS messages in Windows Phone 7 from an app? Does Android and iOS (4 or previous) place similar restrictions on sending SMS messages from an app?
At this stage, the WP7 app can set the number and message using SMSComposeTask.
The user just needs to ok it by sending it off.
This gives the user the opportunity to assess whether sending the sms is worth the cost.
If this were to change, one would hope the user is at least required to set a permission for an app to have unreviewed access to sending SMS. I'd probably want to set a limit on the volume of unreviewed SMS as well.
It's good for the platform and App Devs that users can have confidence that installing an App will not pack a hidden punch.