One issue would be that your messages would be constrained to the lowest common denominator. Twitter means messages can only be 140 characters, and SMS precludes attaching images or links.
As for language/platform, I'd recommend python on Appengine. Easy to develop and it'll scale as far as you want. You can always migrate to a purpose built system when you've proved the concept.
As for language/platform, I'd recommend python on Appengine. Easy to develop and it'll scale as far as you want. You can always migrate to a purpose built system when you've proved the concept.