This is not true at all. I was a loyal T-Mobile customer for almost 8 years, I upgraded my contract, signed another two year contract (at year 6 of being a customer) and 6 months in I asked if they could unlock my phone because I was traveling to Europe and they told me that they don't do that until your contract that is up, and even then it was limited to a small subset of phones.

I ended up selling my phone on eBay and purchasing an iPhone 3G from a friend which I unlocked by jail breaking it!