G4scott, I really think you're onto something here - and I think/hope Apple realizes the opportunity this will give them.
Most windows users (apart from hardcore gamers) are windows users for two simple reasons:
1. They do not know that alternatives exist or don't want to make the effort to check them out.
2. Everybody knows someone from whom to 'borrow' the latest/most expensive windows software.
Now, if Longhorn is not backwards compatible and Microsoft will indeed stop supporting the older OSs to push conversion to Longhorn, what are the consequences for all those windows users?
1. They will suddenly absolutely HAVE TO make an effort to get used to a brandnew OS, either Longhorn - or Linux (not nearly user-friendly enough for the mass market) or, indeed, Mac OSX. There would be no EXTRA effort anymore to check out the Mac side of things.
2. All of a sudden, the abundance of windows software to be 'borrowed' will not be there anymore/useless in Longhorn. "Wait a minute, you mean I have to pay real money for my new software, I should check out the competition then to make sure I get the most for my hard-earned cash. Hmm, that OSX looks rather nice..."
PLUS by then Apple may and we can only pray for that have strengthened their partnership with IBM and thus have obtained some of IBMs cred in corporate environments. Make no mistake, a lot of companys are totally pissed off by Microsoft's way of making business with them, and they only stay with them because the amount of money they already invested in PC hard- and software doesn't allow them to look into alternatives. Well, if Microsoft comes along and tells them to throw away their hard- and software because Longhorn needs faster hardware and/or is not compatible with the software they purchased, they will show Microsoft where to stick Longhorn and either stay on older windows platforms as long as possible or start looking around... "Oh I see, IBM uses Mac OSX on their server hardware now, must be good then..." (There have been rumors about exactly this).
I won't go over the top right now, but it seems very obvious that the moment in time that Microsoft will try to implement a new, modern OS (latest news: Longhorn to use a quasi Quartz Extreme technology, doh) and get rid of all the baggage their old OSs carry with them, will be the very moment in time when Apple, IBM, the Linux crowd and everyone else will get their shot to overcome the Wintel monopoly in the computer world. We can only hope that they will rise to the occasion.