It would be nice for prospective users to know what your advertising policies are.
After that you may want to create a rating plan/level for texters to subscribe to when they sign up.
I know I wouldn't want to see ads for porn or sites that have been proven to spread spam or malicious content.
The concept is kinda interesting but I gotta tell ya', if someone texts me and there is an ad riding along with their message, that'll be the last text I see from them until they unsubscribe from your service.
That means you should probably have an opt out for recipients of these ad laden texts.