No sign of weakness in US iPhone sales, with T-Mobile appearing to be purely additive. There's an obvious diminishing return at work - each new operator sells a lower share of iPhone. This is most probably due to self-selection - the people who really want an iPhone move operator to get it.
Annoyingly, AT&T has stopped disclosing iPhone sales, though it gave some hints. Applying those, it looks like the iPhone was a little over 50% of smartphone activations at the big 4 operators, and 40% of their contract phone activations, which is actually the more relevant metric.
Meanwhile, contract smartphones sales are getting close to topping out. You can't sell more than 100% and the overall contract phone volume is growing very slowly. (Note that this is not adjusted for the Metro PCS acquisition.)