The 13" MacBook, 15" MacBook Pro and the new 17" MacBook Pro are all members of the unibody family, right? This means they share the same design heritage and details.
Now, on the 13" and 15" models, to change the RAM you have to take the large bottom plate off the machine, exposing the RAM, logic board, SuperDrive and all the other internal gubbins. This does not invalidate your warranty, as long as you don't damage any parts whilst changing the RAM.
So, the new 17", part of the exact same family of laptops has the same big panel which you have to remove to access the RAM (and also, in this case, the HDD). This means that removing the panel will be covered under warranty as long as you don't damage any parts when you do so.
Apple are not going to change the warranty on the 17"er to make it different to the 15", as it is exactly the same process. Sure, 4 more screws, but that's because the laptop is, y'know, bigger.
The MBA has nothing to do with this, as it is not user-servicable (RAM is soldered on, HDD is not a "standard" drive). It may be unibody, but it doesn't have the same components.
The 13", 15" and 17" are part of the same family but they dont have the same heritage or internal design build whatsoever. similar, but not the same. The new 17" doesnt have the big panel that the 13" macbook and 15" macbook pro's have which can be removed and tada easy acess! you have to unscrew things and move battery components out of the way to gain access to the RAM and HD. look at the dissasembly instructions on the Apple site.