Multithreading Extensions for Mac OS 9?

P-Rod asks: "With Apple reportedly about to announce multiprocessor G4s, many have been asking why they'd do so before OS X is released, since OS 9 doesn't have the multithreading that OS X has. What about a multithreading extension? Apple has been really good at allowing newer products to work with older ones and vice versa. Apple's ultra-smooth transition to the PowerPC chip back in 1994 is a great example. So is the CarbonLib extension, which lets you run Carbonized OS X programs on OS 8.1 and up. Is a multithreading extension feasible?"