It's about Platform Management, not merely inventory management...
Perhaps I am reading too much into this, but I believe this is more about Platform Management, than Inventory Management, as the latter is a smaller portion of the former, and I trust Apple is acting coherently.
I agree with Amdahl's observation about the user base.
==========
From Amdahl:
They have noticed a big fragmentation in the user base. Despite the big surge 10.5 had the first week, a lot of 10.4 users aren't upgrading. Apple is beginning to have an actual user base(as opposed to a fan base), and that means they can't sell as many newer products if the software companies are busy targeting the older products.
This is all about keeping products going obsolete as fast as possible, and not letting the customers slow them down.
I still hope they will begin a policy of producing security updates for five years after a product is discontinued, but perhaps these discounts are their attempt to avoid doing that.
========
Regarding obsolescence, it is most necessary because it will keep their user base, the Mac OS X platform from fragmenting. However, it is not just, "ALL about keeping it going obsolete as fast as possible." Innovation is one of the greatest strengths of Apple in competing with Microsoft, but Apple is also competing with themselves, in order to encourage us to buy the latest and greatest.
Although at a more micro level, this seems like a shaft to the users, Apple knows that users will not make such high level decisions for the best of the platform and it's future. Meanwhile, yes, they do also want to make a profit. However, their job is to create a compelling enough reason for us users to continue buying. I don't mind this. If Apple puts out crappy updates and upgrades, I can speak with my dollars. The trick is, keeping those updates and upgrades compelling enough, even in a down economy. If they do this well, they keep their user base unified.
Unified user base will have many positive benefits, perhaps the only drawback is greater susceptibility to malware. Interestingly, M$ has a huge problem with unified and fragmented user base, and malware due to their size.
So in the end, it is only secondarily about inventory management. I'm not sure if Apple uses a push or pull model for inventory, but as they keep a pretty timely supply chain, that becomes moot. It seems this move is either a small inventory management tactic, or perhaps even a strategic one, coupling keeping the MobileMe base current while also encouraging early adopters, without a huge commitment from Apple. Finally, it may be a small attempt to unify the base shortly before Snow Leopard is released. Of course, all these reasons may not be mutually exclusive.