Upgrading the HDs will Void your Warranty, if you attempt to do this yourself, since this is not as easy. Also, Apple specifies that if you attempt to make an AppleCare claim and find a non-Apple certified HD, they can refuse service.
I know what your saying, but on my MBP, when i sent it in for them to Repair my IR port on my laptop, they refused because the prior owner changed the HD to a non Apple Certified drive, which does work. I'm on it right now.
The previous MBP's hard drive wasn't user-serviceable. But on the plastic MB and all the unibodies, the HDD and RAM are user-serviceable (except for the Air).