Correct. If you unlock your phone, that just means that you can take that phone and use it on another network using another networks SIM. It is basically for if you wanted to switch providers, from Tmobile to AWE or Cingular and still continue to use that phone. It would also be the same if you unlocked it and took it to Europe and got a prepaid SIM over there, you would be able to use it with your new SIM.
Tmobile must have a roaming agreement in place to use other networks, and there are only select places where you are able to roam. However, Tmobile has coverage enough for most people. In the places that they do offer the ability to roam there is no roaming fee on a national plan in the mainland US and Hawaii, so roam all you want to and you will be good to go .
Courage is being scared to death...
And saddling up anyway.