I've heard today that owls cannot hunt in the rain, is this true? If so, then why?

It is true that owls are reluctant to hunt in the rain, mainly because the feathers on their wings can become waterlogged very easily. I don't know about other predators.

As a follow up to Xeract's post--the reason the owl's wing becomes waterlogged easily is due to the soft fringe along the one edge of the wing feathers which enables them to fly so silently. It absorbs water very well.

Kingfisher