Wow - I have a 21" imac and did this based on everyone's advice and it worked! Thanks for the scoop. I could have easily wasted three hours taking this to apple store or calling apple care.
My method from the other responses was to take a manila folder and cut it into an "L" shape about 4" high and 3" wide. I then turned the computer off. I then turned the iMac on it's side so the dvd was facing down. I left the iMac stand on the desk and then rested the side of the iMac on my legs, but there could be other ways to turn it sideways that leaves the slot accessible. I then fished the L shaped manila folder piece into the slot (with the leg of the "L" facing the bottom of the iMac). I did gently shake the imac on my leg a little bit, and also made sure that the iMac screen was perpendicular to the floor - not leaning one way or the other. I then pulled the L down slowly and not too hard. Sure enough, on the 3rd try or so, the sd card pushed it's way through the spongy material at the dvd opening. Couldn't believe it! Thanks everyone.