i feel like i should weigh in...
It would be much more logical to take pains to keep the iMac in it's current (standard) orientation. First off, the physical design of the case means that heat will more than likely build up on the "up" side. If you place it with the ports near the bottom, this will heat up the ODD, increasing the odds of overheating/failure. If you flip it the other way, the logic board (containing CPU and GPU) will get WAY too hot. Also, like Hellhammer said, forcing the GPU to flip everything like that will make it work harder than it has to. I suggest mounting it vertically.
Good Luck.