Microsoft has officially announced that Xbox Live region migration is a go. This means that users moving to a new region of the world, say from the US to Europe, will be able to take their "gamerscore, unlocked achievements, any Microsoft Points already in your account and any remaining time on your Gold membership" with them.
Of course, when in the new region, some things will be different. For instance, payment methods, content, and select services may not be available in the region you have moved to, but this doesn't come as a surprise. When you move into that new region, your account then falls under the rules of that region.