It was only a matter of time before this happened, but the T-Mobile MyTouch 3G has been rooted, i.e. cracked wide open, meaning that it's ready for all kinds of customization that the stock Android installation keeps out of reach, including cooked ROMs.
In real terms, this means you'll be able to port the Hero's fantastic Sense UI to your MyTouch, as people have been doing on the G1 and the virtually identical HTC Magic for months. Mandatory disclaimer: this could brick your phone, so tread carefully. [HTCPedia via Crunchgear]