Before you start thinking – why can’t all AT&T phones be unlocked and used on T-Mobile in the same way, it’s because the Galaxy Note has the necessary HSPA+ access built-in, just that it is configured to different bands hence the need to hack the phone before it can work.
If you’re interested in turning your AT&T Galaxy Note into a T-Mobile HSPA+ compatible device, the instructions are available on XDA-developers. As usual, remember to back up your device and make sure you know what you’re doing before attempting anything. You never know what could go wrong.