I guess it's a nice thought, but I don't see it. Retail boxed OS and software made up less then 1% of Apple's revenue in the current fiscal quarter - Mac's were about 58% of revenues (the majority of those being laptops), and iPod's and Music products (including the iTunes store) made up nearly 42%. So, say opening up the OS reduced hardware sales by 10%, then, at $120 per OS copy, Apple would need to sell over 2.3 million retail copies of OS X to make up for the hardware shortfall in revenue (based on Q4 2006 revenues of $4.84 Billion US).
I just can't see that happening. OS X really only exists to keep people buying Macs - the same as every version of the Apple OS before it.
And by tying the OS to the hardware, Apple does produce a better overall product, IMO, with better consistency and fewer issues then many windows platforms (and, just to be clear, I have, and do, own quite a few of both).