I was having the same problem with my MBPro (17in early 2009) I had went through all the listed necessary steps. Upon examining the logic board I had noticed a tiny gold ribbon that was unsecured [on the lower left side (near the cd drive) near the battery] I put this ribbon back in place with tweezers, closed up the Mac and Voila! I was able to put my Mac to sleep just by closing the lid.

replaced top case - magnet on wrong side
 

Thanks for all the discussion on this - further investigation on my own led me to find that when I replaced my top case I used one that is identical to the original in every way, except that the magnet in the LCD frame is on the opposite side! unbelievable... I guess it's time to bust out the dremel and install it in the right place.