I'm pretty sure you will not void your warranty. This is printed right in your manual:
"If you open your iMac or install items other than memory, you risk damaging your equipment. Such damage isn’t covered by the limited warranty on your iMac."
As you can see, it does not specifically say that you "can't" do it, just that if you do open it and do any damage, they won't cover the damage. The new iMacs also have a "warranty void" sticker covering one of the screws on the heatsink inside the case. I would assume that sticker should be on the outside of the case somewhere covering a screw if it was going to void your warranty just by opening the unit. That's the way it was on my Asus Netbook (1008HA), as soon as I popped off the keyboard there was a warranty void sticker covering one of the screws you needed to remove to continue opening the unit. Case closed. (pun intended)
And the following line is from the iMac warranty doc:
This warranty does not apply.... (f) to damage caused by service (including upgrades and expansions) performed by anyone who is not a representative of Apple or an Apple Authorized Service Provider (“AASP”)
Again, it only specifically mentions that they will not cover "damage caused", not a blanket statement suggesting your entire warranty is null and void if you do an upgrade.
I'm sure you'll get lots of opinions on this so read it as you will but as I read it, in my opinion, as long as you don't damage anything then your warranty is still good other than removing the heatsink which specifically has a "warranty void" sticker. If you damage anything then they reserve the right to charge you for repairs to fix the damage and of course they won't cover the new drive either. Also, if something goes wrong with the system down the road and they can prove it was a result of your upgrade, safe bet they'll charge you for that one too.
http://images.apple.com/legal/warranty/docs/cpuwarranty.pdf
James
P.S. Shameless disclaimer: just a reminder that I'm not a legal expert! This info is my opinion only so you're on your own if you do the upgrade. Also depending on what country you're from, you may also have additional rights that aren't covered in the warranty doc.