Well, I gave them a couple of days to jump in here...
So I'll just pass along what Brent told me;
(Sorry Brent if this is out of line!)
Below is my question to him, and his response;

TjB; "So, Are you guys all sitting around trying to decide what you are going to tell us about the change in drivers in the EP350, or are you coming up with a new name for a new sub?"
Brent; "You are an astute man.
The driver is a temporary replacement and will not be ongoing in the EP350. Essentially it is the same driver without the dust cap."

Pay particular attention to the part that says this is temporary, and will not be an ongoing replacement.

My guess is that they ran out of the 350 drivers, and decided it was better to ship subs with the 500 drivers rather than enduring a long out of stock, and possibly losing some orders. Perhaps they are/were going to be out of the correct drivers for quite some time?

And, I also do not really buy the "Essentially it is the same driver without the dust cap." thing.
If it were "essentially" the same driver, then why would they bother buying both?
I simply cannot imagine Axiom undertaking a project like the EP500/600, with all of the technilogical innovations that it included, and then just using the same driver from the previous generation subs. From what I have able to glean from the several months of research I put into buying these speakers, and the short, but involved time I have spent on these forums, this just does not seem like something that Axiom would do. Does it?
I certainly do not want to put words in anyone's mouth at Axiom but I think they simply ran out of the 350 drivers, and thought, "Hey, the 500/600 driver will fit, and some guys will get a little bit of a bonus, even if they don't notice! No harm, No Foul..."
What they might not have counted on (which I would have thought they would have seen coming from a mile away, considering these threads, and the folks who post here)was someone noticing, and then posting that observation, with the predictable results of those who got the 500 driver Crowing over their unexpected windfall (Guilty as Charged - Sorry!)and leaving those who did not get the new driver wondering if they got gyped - when they actually got exactly what they had ordered.
In the end I think that Axiom, being the great company that they are, with, (in my experience, and by EVERYONE'S accounts here), a tremendous record of customer service, came up with the best and quickest solution to see that their customers got their precious new subs in a timely fashion, and like so many other Good Deeds, it has come around to bite them in the behind!
Personaly I kind of feel bad that they are in this sticky little situation, cause they all seem like such nice folks, and certainly do not deserve ANY bad Karma in any form what-so-ever. And I kind of feel guilty about being a part of it all...
But I sure Ain't sending my "Experimental EP400" back!


Axioms; For when you've just Gotta have More Cowbell! 60s 150 350 8s 2is RX-V2500 DVD-C750 2900