Some flowering paniculatas are notorious for attracting bees to the point that some people plant the shrubs away from high traffic areas. I wonder then, could the cutter bees be stealing the leaf material and the other bees be the ones actually damaging the sepals?

Leaf cutter bees will sometimes take their material from petals. I would think petals larger than hydrangea but I really don't know. If you google 'leaf cutter bee flower petals', images, you'll find some examples of roses, other.

Are you seeing the same perfectly cookie cutter round as on the leaves?

Here's a picture. The holes appear on the bigger blooms as well, but the leaves provide better contrast in this shot. I know the leaf damage in this particular picture is not cutter bees.

That really does look like leaf cutter bee activity to me. They must really, really like your hydrangea. And you do realize, they may never choose it another year but this year you appear to be twice chosen :)

Thanks morz. The only information I could find about cutter bees damaging the petals said that they do this when the flowers are still closed in order to get to the nectar inside. So it's possible this happened at the nursery before I purchased the plant. There are still plenty of flowers on my 3 limelights so I'm not too worried about their appearance. I'd just never seen that before!