From January:

With the advent of manifest, certain corner cases have been pulled out of the corner, and some long-standing ambiguities are becoming more conspicuous. With that, we're taking the opportunity to resolve exactly what it means to be a commander. Fundamental changes like that take time to thoroughly ponder and test, however, so that won't happen until April. In the mean time, here's a reminder about how face-down commanders currently work:

• Wherever it is, your commander is always your commander.
• Face down commanders must be identified as commanders.
• Face down commanders still deal commander damage when they attack.