I've heard about this too - opt in rather than opt out is an excellent idea. I actually spoke to someone recently who works in the SMS industry who thinks it's a good idea to prevent abuse of the system by unscrupulous companies and yet still allow legitimate ones (like the one he works for) to market products.

As for ISPs being liable for chat messages - I agree when they say that they are similar to the postal service. So long as any inappropriate messages are removed within a reasonable time of the ISP being notified (although what is reasonable would have to be defined by the courts) and the ISPs don't openly allow or encourage malicious/defamatory messages to be opsted, then it shouldn't be their problem.