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But there's a difference between supporting a third party player and simply not interfering with a third party player connecting. Neither Microsoft nor Dell will "support" the off-brand Chinese sound card you install on your computer, but they also won't forbid you from installing and using it as long as you can make it work. (and by support I mean if you call them and say your sound isn't working they'll tell you to take it up with the manufacturer of the sound card.
Apple would have no responsibility to support the Palm Pre. They could just say "go talk to Palm about it" and that would be standard industry procedure. They already do it every day for hundreds of Mac accessories and peripherals.