People had been saying for awhile and, correct me if I am wrong, but I think even the Source said [for awhile] that Wii would pass the 360 and then shortly thereafter 360 could take back the lead.
I was thinking about this scenario today and by calculating things in my head I figured the Wii would have a lead of about 700k which is damn close to your guess of 800k when Halo hits. I don't know what sort of hardware boosts the original Halos gave, but with no support from Japan and only slight support from Others, it just does not seem possible that the 360 could ever take back the lead once it is gone. The 360 would have to sell 500k units a week for three weeks straight just to take it back and that seems ridiculously high to me. I think come August 19 we will be seeing the Wii in the lead indefinitely and most likely permanently.
As for industry implications I don't see it being huge. I am not sure, but I don't think Nintendo will announce anything at all until October when shipment numbers come out because otherwise it is just too unofficial, but that's just my opinion. I think the industry trend is already playing out of developers switching resources and that will just continue to snowball. I think the biggest impact it will ever have is people at the forum will have a big party 
