I was wondering if I buy a MacBook Pro and replace the preinstalled OS X installation with Ubuntu, would that void the warranty?
no but if you have any technical issues with the computer you won’t get any useful help from Apple until you reinstall OS X. Dual booting might be a better choice.
That would be a silly waste of money. The resultant installation will function sub optimally since the hardware is optimized for OS X systems and Apple has never provided Linux-friendly (or compatible) libraries that can be used. All that's out there is the product of unsanctioned reverse engineering and is not guaranteed to work in current or upcoming models. This, BTW, applies to dual booting as well, for the very same reason: BootCamp only supports current Windows versions.
In other words, you would be making a much wiser investment by buying another brand that guarantees full Linux compatibility. And might even save a nontrivial amount of money as well.
Does deleting Macintosh HD void MacBook Pro warranty
