Quote Originally Posted by tehlegend
the last element has something to do with yin or yang nature. yamato mentioned it in nauto's fuuton rasengan training session right after showing off and making that waterfall... i would be surprised if they held out on going into detail about that beyond this, especially at the mention of the 6th element.

I think that is the most likely possibility as well. I remember when it was first mentioned I was thinking that this was a major development that would affect the way that future battles went much like the introduction of elemental strengths and weakness but it was passed over so quickly that I completely forgot about it. Now that its been reintroduced especially since it was explicitly stated that Minato sealed only the Kyuubi's yang chakra within Naruto and then there is the sudden and random mention of a sixth element, I think they must somehow be related because there is no such thing as a coincidence in Naruto.