Then you heard wrong. The magnetic permeability of aluminum is the same as air (or plastic for that matter). There is no magnet in the Aluminum iMac save for the ones that hold in the glass*, some of which may be strong enough to hold the remote.
* Technically, the speakers also have magnets in them.
Oh. I remember a bunch of disappointed MacRumors ppl were frantically trying to find it when it came out and that rumor surfaced. lol. I guess MacRumors is the best title for these forums!