@andstatus As I mentioned, there is already a global user identifier. The URI for users will always be the same across profiles. And profile URLs all have metadata for various methods of discovery of this URI (usually it ends up being the /user/$id URL).
If that data is not part of the Twitter API then we can arbitrarily add it to ease API usage. But the Twitter API sucks and the ActivityStreams (.atom and .as endpoints) are the ones that give proper, federation-usable data.