How popular is the iPhone? Popular enough that over a million T-Mobile customers have taken unlocked iPhones, trimmed the T-Mobile SIM card to fit the phone in the case of iPhone 4s, and are actively using them on a network that doesn't support them. That is nothing short of amazing, and proves how popular the iPhone is compared to other smartphones.
These unlocked iPhones don't get 3G or 4G data speeds because the radios in the phones do not work on the T-Mobile network, which makes this even more amazing. Apparently these iPhone owners get what they need from the phones without support from the carrier. While you can argue that a few T-Mobile customers might do this not knowing what they are getting into, it's hard to believe that over a million folks did this without thinking things through.
While T-Mobile doesn't handle the iPhone currently, maybe that will change if the AT&T merger gets approved. Imagine what these customers will think if the phones work on the network as designed.