"there is nothing to take seriously"
This attitude is either mindless bravado or some kind of denial. When has Microsoft ever decided to enter a new market then failed to take a meaningful chunk of that market? Seriously. Name me one example and for every one you name, I'll name you a dozen on the other side of the ledger.
Did Microsoft have to eclipse Quicken with Microsoft Money to have a financial impact on Intuit? Did they have to run Playstation or Nintendo out of the gamebox market in order to impact their finances?
As roni pointed out, it's silly to approach this as an all-or-nothing paradigm, where the only way Microsoft has meaningful impact on Apple is if it somehow surpasses and eclipses Apple's market share. Set aside this "win vs. lose" thing where Macolytes and Windowphytes play their childish games of "good" and "bad" and "win" and "lose" as mutually exclusive events. If Microsoft takes away 10% or 15% or even 20% of Apple's share, that's going to hit Apple's top and bottom lines, and it's going to impact AAPL. How much of an impact? Now that's something worth exploring and debating numerical hypotheticals.
Do you really need to see a locked in feature set and price and pictures and a retail box in a store before you believe Microsoft could take 10% of Apple's share? That would be almost as assinine as Sony Playstation nuts declaring that Microsoft didn't know squat about the gaming business, and there was nothing to take seriously until Xbox was shipping.
It doesn't take a great deal of vision to know the general shape of how this will roll out. Apple WILL suffer an impact. The question is how big the impact will be in the first generation as Microsoft stumbles around, finds its footing and balance and learns by trial and error. And then how much larger the impact will grow and at what rate it will be expanding.
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