The curling is a natural response of rhododendrons to the cold. It varies within a plant and from plant to plant. It typically starts below freezing and stops above freezing. Around freezing it depends on the hardening (salts and sugars in the plant cells) and the state of dehydration.

I have some plants with leaves that I don't expect to uncurl. I think those particular branches have suffered terminal damage and I will prune them off when I am sure they are dying. We had a very mild winter followed by a month of bitter cold. I think this unusual circumstance tricked some plant parts into breaking dormancy before the bitter cold and hence the frost damage.

If the sap starts rising in a plant too early, freezing causes bark split and usually death to the branch involved. I haven't looked yet to see if I have bark split.

Here is a link that might be useful: Henning's Rhododendron & Azalea Pages