Microsoft has launched ads specifically mocking Apple’s ads (which themselves mocked Microsoft) and Ballmer has been quoted bad-mouthing Apple more and more often of late (most recently and somewhat bizarrely, denigrating Microsoft’s own Office for Mac).
For a long time, Microsoft made more money on each Mac sold than on each PC sold. This is probably because most Mac users bought a rather expensive copy of Microsoft Office whereas most PC users got theirs bundled or used copies from work (which Microsoft allows, indeed probably encourages to prevent competitors from having any chance of selling anything to anyone). I doubt this is still the case since many Mac users get by just fine with the cheaper, more pleasant, and for many purposes superior iWork.
It’s sad for Microsoft that it’s attacking Apple just as Apple gets to the point where it really doesn’t need Microsoft for anything. If Microsoft, for example, threatened to stop developing Office for Mac today, Apple would probably just laugh at them. OpenOffice.org provides a workable replacement for Office if you desperately need an ugly but serviceable Office suite, and if Apple wanted to, it could throw its UI prettification team at OOo, or it could simply add a few features to Pages and Numbers (Keynote is already, effectively, on par with Powerpoint).
Meanwhile, iPhone has been making WinCE … oh wait I mean Mobile Windows or whatever it’s called today … look silly, and Apple has stolen a huge share of the “PC market that actually makes money” leaving Microsoft’s cohorts to fight over the 2% margin Walmart market. If Apple makes a couple of fairly obvious moves with the App store and iPhone/iPod Touch games market, it could make Microsoft’s money-losing foray into the console games market look pretty sad too.
Declaring war was probably stupid at any time, but declaring war when you’ve already lost is really stupid.