Look at the situation with the iPhone: it was only available on one carrier, and customers pretty much revolted and unlocked their iPhones.

Problem is that providing a great experience isn't the same thing for everybody: for some people it is having an iPhone, for dome it is having a Treo, for some it is connecting their EEE PC with 3G, for some it's android, for some it's blackberry etc... and it is the same thing for online services.

If carriers want to control what devices get on their networks they either need to support pretty much any device (at which point they lose control anyway) or face user discontent, leading to high churn rates, early terminations, then government intervention because of the early termination fees etc... How many people left their carriers just because they couldn't use an iPhone on their current networks?