There is a Mears shuttle you can arrange for that will pick you up at the resort, for I think, $15/person round trip.
We decided to go with a cab, because we didn't want to be stuck with a particular pick up or drop off time. Plus we wanted to be returned to Downtown Disney. It was a good thing too, as the day we went to IOA, there was no one there (DH went on Dueling Dragons 16 times!). We arrived at about 11 am and expected to be done around 5 or 6. We were ready to go a little before 3 (like I said...no one was there

).
It was about $30 each way.