Not impossible... we'll have to wait and see...
If this happens at all, it will go something like this, so says TheWama:
IBM has a strong incentive to expand it's share of the chip market, which is currently stuck mostly to server-level applications.
Apple has a robust, impressive consumer-level OS, but is, for the most part, ignored by businesses, despite respectable server-level products. They are also constantly hampered by the perception of being a niche OS, and the implications that carries for third-party software development.
If IBM licenses Apple's OS for corporate PCs, this will mean added cashflows for apple from a market they weren't even competing over. This will also mean adding an air of respectability among the corporate market that IBM has and Apple, for the most part, does not. Greater market share for OS X, that comes without canabilizing Apple's consumer PC sales, is a huge boon to Apple.
IBM, in return, gets greatly increased PPC sales, because they can now market it to the corporate workstation crowd, as well as supporting Apple's sales. This can take the form of a joint project between Apple and IBM.
It does *seem* to make sense for Apple and IBM, based on the little I know about the situation. Whether it actually happens, well, we'll have to wait and see.