Actually XP on Mactel is quite likely to happen, won't be supported by Apple -- because that type of phone support is quite costly.
Likely Apple will work closely with MS to make sure it happens, and any Windows support needed will be paid for by the user.
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OS X on all PCs means Apple is competing directly with MS, and that is a deadly place to be.
As long as Apple tries to make sure it doesn't happen, MS will likely not lob the nuclear lawyer on Apple.
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If you want OS X, buy a Mac ...
If you like the Mactels, buy a Mac and run Windows.
Apple will increase their sales base and become one of the fastest growing PC makers soon, but not bear any costs associated with supporting Windows.