There's not a fixed date for the Nats.
In theory, the 2011 Nats could be held on the 31st of December, and the 2012 Nats on the 1st of January and it would bo OK.
The other way to look at it, is that the 2008 Nats in Tassie were in early June, so the 2008 title holders had their titles for more than 12 months.
Whatever else you think of MA, I think its grasping at straws to claim that they're trying to undermine the 2009 title holders.
Similarly, I don't think that they're trying to "undermine" Classic Dirt - although they're clearly 'sharing' the basic concept, there's zero evidence that MA/BBB is trying to bury CD. I mean, if that was the motivation, then there's a ton of far more agressive things that MA could have done (closer date, make it cheaper/easier than CD, more dirt-only focus, etc).
I'd argue that having BBB and the Nationals within a week of each other (in April) actually offers CD7 some security. It is entirely possible that either BBB or the Nationals could have been held on/nearer to the June long weekend, which would have had far more of an impact on CD.
Don't get me wrong: I have questions about some of the things MA has done (and particularly the process they used to arrive at those decisions), but luke-warm conspiracy theories only serve to distract from the real concerns.
(Sorry for the rant, I've just been watching the conspiracy theories gain momentum and I am afraid that we'll all have to start wearing tin-foil hats before CD7).