@The Source
Let me say this Source, and I think it simplifies it a little too much, so take it with a grain of salt:
The Wii, at this point, in the Americas has nowhere to go, due to supply constraint. Assuming that analysis is correct, and the trend continues, the only control Nintendo has over the time period in which is passes the 360, is shipment amounts.
The 360, on the other hand, has so many variables that are unknown(as I think you oversimplified them in your original post). New SKU, probably price drop, adoption rate of the console with GTA, supply issues, and the possible but unlikely delay of GTA.
So, the obvious answer is we don't have enough information to analize this correctly, but we can make several assumptions:
1. If the 360 has a price drop, or a new SKU, to coincide with the GTA launch at a point earier than May, we're likely to see vastly increased 360 sales in the Americas.
2. If Reggie claims the Wii will surpass the 360 by June, it indicates that he predicts increased Wii shipments month over month, from January NPD.
I wouldn't believe that the Wii would pass the 360 by August, were it not for Reggies confidence in his statement.
My trust isn't in the man himself, as much as my assumption that he must be privy to information that we are not.
Either way, I think its safe to assume that Nintendo wouldn't lower the price of its system for something as petty as besting the 360 in NA. I think that the fact is clear, the Wii will not offer a price cut until it is able to meet demand in all three major markets.
That said, I realize your theorization was on HOW the Wii could pass the 360 and not a prediction.
Thanks for posting, I enjoyed reading it.
Final thing, the Wii does have one big 3rd party console pusher in 08, but its only for Japan, in the form of Monster Hunter 3.