Hello Everyone. My friend also the gray streaks on an iMac 24. We popped off his glass and notice the were lots of streak on the backside of the glass as well. Maybe from smoking....IDK. but I can tell you that I cleaned the glass using MONSTER SCREENCLEAN Home & Office Display Cleaner and I used the little black cloth that came with his iMac. I must say that is the absolute best screen cleaning solution combination that I have personally used to date.
Now the gray streaks on the display itself are still there and he will probably have to have that replaced but as far as the glass cover is concerned it looks brand new now.
I know this doesnt solve all the problems, but if you just need to clean the glass please give this a try. You can find Monter Screenclean at Best Buy...probably by the TV section.
P.S. He was so happy at how bright and vibrant the screen looks again (don't forget this mac is about 3yrs old) that they gray streaks behind on the display itself dont even bother him anymore. Although i would personally probably want to have that replaced if warranty covers it but to each their own.
If you don't know how to take the glass off there is a good vid on youtube about it. I can tell you that two small suction cups on the top about 2" in from the left and the right are good places becuase thats where the guide pins are on the top of the screen.
GOOD LUCK!