Hadn't reported a while back:
Having successfully replaced one of the 2/ea 2 GB boards in my Spring '08 iMac with a 4 GB== 6 total--with no discernible ill effects.. [Rated: 4 GB Max] ...
Later on, a sale at ~ half what I'd paid for the 4 GB and despite iFixit (or similar folk) nixing "beyond 6", for that exact model: I rolled dice. (Other models Had reported varying results, so.. wtf.)
It Did 'recognize' the full 8!
BUT.. thrashing (and perhaps as Alex suggests: result of absence of extra mobo address lines? for all pecuniary reasons) and.. it ran really s l o w ==> doom.
Passed along the board elsewhere, etc.
(I'd believe that firmware may be part of the obsolescence scam. Because it's so Easy.)
But if someone will take-back the stick.. 6 sure beats 4; want 16 too!