Do Not Make Repairs Yourself
Your iMac has no user-serviceable parts, except the keyboard, mouse, and memory
(see “Installing Memory” on page 39). Do not attempt to open your iMac. If your iMac
needs service, contact an Apple Authorized Service Provider or Apple for service. See
“Learning More, Service, and Support” on page 62.
Apple's warranty states:
"This warranty does not apply...if the product has been modified without the written permission of Apple..." Adding memory (DRAM, VRAM) or other user-installable upgrade or expansion products to an Apple computer is not considered a modification to that Apple product. Therefore, it is not necessary to obtain Apple's written permission to upgrade or expand an Apple computer.
Just to add an interesting tidbit: I had occasion to watch an AASP dismantle my last iMac in order to replace the logic board. In any case, I watched and noted the following tape on the hard drive: "warranty void if removed". It was in a spot where it would have to be removed to get the hard drive disconnected.