I suppose
this is the iFixit guide you're referring to.
I have lots of experience opening/putting together electronic devices, but trying to replace a worn out CCFL backlight in my Powerbook G4's screen was a rather unpleasant experience as I wrecked a perfectly good LCD display. Then again this involved actually opening up the LCD component itself! How stupid not to make those backlight easily replaceable, but then again they'd miss selling a replacement LCD (that's green thinking for you!).
I assume inserting a 2nd drive in the iMac is nothing as risky as that though.
Is it a challenge to perform the "surgery" in a way that won't leave any traces of "tampering" in case of warranty issues?