How will TMo users get 3G roaming worldwide?

I have lurked here for ages but this is my first post.
Sorry if someone has asked this elsewhere but I have a question.

I am a TMo subscriber watching and waiting for the Bold/9000 to be released. I know that ATT will get the device first in the US (eventually) and that in the interim some people are buying from Rogers (or CLS in HK etc) and using the unlocked device in the US on ATT and on TMo (without 3G).

From what I understand the TMo 3G network is on a different frequency to Rogers, ATT and others and an unlocked ATT Bold (when it is released) will not get 3G service if used on the TMo network. To get 3G on TMo network you will need a Bold/9000 that has been configured specifically for that network.

And so my question is this: If I wait until TMo offers their specially configured Bold on their network how will I be able to get 3G in other markets globally when I roam if all these other markets are using a different frequency?

Will ATT subscribers have access to a higher number of compatible 3G networks internationally? If I stick with TMo am I going to have a device that cannot access 3G in other markets because I am on the wrong frequency?

I travel a fair bit internationally and am wondering how access to 3G speeds in other markets while roaming is going to work.

Thanks for any insight or ideas.

Mike