In case we needed another reminder of why it doesn't matter that the iPhone can't multitask, a study published in Applied Cognitive Psychology indicates that most of us can't, either.
Specifically, 75% of people walking down the street while talking on their cell phones did not notice a unicycling clown passing them. (Oddly, neither did 50% of the people who were not talking on cell phones, so really the phone conversation only cuts our attentiveness by an additional half.)
Read about the study inScience Daily
So even if we could take photos or video while on a call, chances are good that we wouldn't notice the opportunity. In any case, please be careful when walking, biking, or driving, consider pulling over to take a call, and watch out for clowns!