If you've ever seen the innards of an ultrathin computer such as a MBP, you'll know there's not much free space inside. Coins pushed in the slot won't have room to move anywhere else but within the confines of the drive itself and once they're in, I doubt that you would be able to get any CD or DVD inside as well. However, due to the anti-dust brush at the edge of the slot, getting them to drop out is near impossible, so the drive will probably have to be taken out to shake the coins out. It is also very probable that the drive mechanism may be damaged by the coins, requiring a replacement.
IF the computer is out of warranty (regular or extended Applecare)
AND you are dexterous and up to the task, you can find take-apart manuals [right here|http://www.powerbookmedic.com/Take-Apart-Repair-Manuals-p-1-c-258.html]. Otherwise, let an authorized and trained pro take care of it.