I'm not sure if this counts as an "official line", but there is a short YouTube video of Ken Schwaber and Jeff Sutherland which touches on this matter: http://www.youtube.com/watch?v=CAx2haAodY8
If we consider past experience (and in agile practice we usually do), we should expect that there'll be new organisations and certifications in the future. An agile practitioner should have a broad understanding of agile methods and their evolution, including XP, DSDM, Lean, Kanban, and TDD as well as Scrum. Don't be tempted into making a narrow choice. The present differences between Scrum.org and the Scrum Alliance are a mere bagatelle in the scheme of things.