T-Mobile revealed a month ago that  it had over one million iPhones running on its network. A surprising annoucement because the iPhone officially didn’t support T-Mobile’s network and the phone wasn’t able to take advantage of 3G data speeds due to differences in the carrier’s network standard. Before today’s announcement, T-Mobile customers who wanted to use the iPhone on the network had to trim their T-Mobile SIM cards to fit into the iPhone’s micro-SIM slot because the carrier wasn’t offering smaller cards.

First spotted by App Advice  , T-Mobile USA has officially started to sell micro-SIMs which are compatible with the iPhone. T-Mobile USA is specifically targeting iPhone owners on its page promoting the new cards.

Already have a GSM phone or iPhone that you need to activate—or have an unlocked GSM phone you want to use on the T-Mobile network? Just get a Micro SIM card and choose a plan. Slip the ready-to-use Micro SIM Card into your phone and you’re ready to go.

The micro-SIM is free through T-Mobile’s website if purchased with a two year contract via the website or $9.99 without a contract. However, even if you decide to use the iPhone with a official micro-SIM from T-Mobile, it will still not be compatible with T-Mobile’s 3G network.

Rumors have suggested that Apple is testing a T-Mobile USA compatible iPhone, and will offer it to the public at some point. In addition, a T-Mobile compatible iPhone is even more likely since AT&T and T-Mobile are currently in the process of merging.