From the iMac User's Guide:
Quote:
WARNING: Do not attempt to open your iMac, except to install memory. If your iMac needs service, see “Learning More, Service, and Support” on page 57 for information about how to contact Apple or an Apple Authorized Service Provider for service. Your iMac doesn’t have any user-serviceable parts, except the memory.
OWC aren't "users." They're Apple Certified. So if they're doing other things that Apple Certified Technicians are allowed to do without voiding your warranty (hard drive replacements, repairs, etc), it won't void your warranty.
Of course, if OWC replaces things for you, Apple isn't going to warranty those parts, because you didn't buy them from Apple.
An eSATA install & case mod, on the other hand, is
not something Apple Certified technicians are ordinarily allowed to do. So that's almost definitely going to technically void your warranty. You might run into a super-nice/awesome Apple Technician who'll choose to overlook it as long as it's done properly (which is what OWC would do), but it's definitely
grounds to not honor your AppleCare, for sure.
Basically, Apple considers the warranty valid on the machines they sell, and the options they sell them with, not on "anything that this machine could possibly be made to do." Which makes sense, frankly. If you were a cook, and you guaranteed your food wouldn't make your customers sick, you wouldn't extend that guarantee to other things the customers put on/in your food.
In simple terms: the eSATA thing specifically means you're taking a huge warranty risk by doing this. OWC's work isn't shoddy, but don't count on anyone else footing the bill if something goes wrong in the future, even if it's totally unrelated to the eSATA install. Personally, I simply wouldn't do it unless my machine was already out of warranty, or I felt comfortable doing any future repairs myself.