While Apple has launched the iPhone 4S on AT&T, Verizon, and Sprint, and is even about to launch on regional carrier, C-Spire, T-Mobile says they're still waiting for their turn, and for a version of world's most popular handset that runs on their rare AWS high speed frequencies.
T-Mobile thinks the iPhone is a good device and we’ve expressed our interest to Apple to offer it to our customers. Ultimately, it is Apple’s decision. The issue remains that Apple has not developed a version of the iPhone with technology that works on our fast 3G and 4G networks. We believe a capable version of the iPhone for our 3G and 4G networks would offer an additional compelling option for our customers on a fast 4G network. However, the iPhone is not the only option to experience the benefit that smartphones offer.
Note: When T-Mobile talks about 4G, they mean HSPA+, not the truly next-generation LTE. Like several smaller carriers in the US and Canada, T-Mobile operates their higher speed network on the AWS frequencies, which means the current iPhones can't run on them. (An unlocked iPhone on T-Mobile can only run on their older, slower, 2G EDGE network.)
Maybe Apple doesn't want to complicate their current one-model-rules-them-all strategy of a single GSM/CDMA world phone, and simple adding AWS to the radio stack isn't an option, or maybe they're waiting to see how the AT&T-Mobile merger pans out before deciding.
All we know for sure is that T-Mobile customers remain among the increasingly smaller group without access to a fully functional iPhone on their carrier of choice.