I'm a bit confused about this topic.  I have a T-Mobile myTouch 3G which I've recently replaced witha Nexus One so it's kind of standing there.  I was contemplating rooting the myTouch and installing an alternative ROM but I also wanted to unlock it so it can be used with other carriers.

Now, the real questions:  When rooting and installing a custom ROM, will that also unlock the phone so it can be used with other carriers?  Or is that a baseband feature?