As much as I enjoy the thought of the Wii winning the past month so clearly, and how I usually accept the supremacy of NPD, I'm going to have to side with ioi on this one. NPD may have the larger market coverage, but every single analyst and preliminary bit of data was in favour of VGC. On top of that there's all the weird inconsistencies like Brawl selling way more than even Nintendo reported at first and then droping like a rock.
There's also no way to get around how HD console sales dropping despite the launch of GTA4 is utterly preposterous. It goes against everything we know of how the industry works, and every single source on the planet was reporting an increase in sales.
NPD do this stuff for a living, and they have a reputation to uphold. One of the biggest reasons I like this site is how ioi never hesitates to change his numbers, despite how some think that makes him less credible.
I doubt they consider VGC so important that they'd fudge their own numbers just to discredit us, but maybe they've realised how some of their other numbers were wrong, and are now fixing it by changing their later data? That's the only reasonably likely explanation that seems even remotely feasible to mef, other than sheer incompetence on one or both sides.
Btw, I remember we had a similar controversy last December, and didn't VGC numbers turn out to be more accurate then when compared to shipment data straight from Nintendo and the rest?