Karen, when you planted, did you loosen the rootball before installing it in your bed? Often rhododendrons purchased in containers are quite root bound, and its easy to water the surrounding soil while water isn't entering the rootball of the plant where its needed.

I can't quite tell from your picture either, is it planted no lower than it was growing in the pot? Rhododendrons have a dense network of surface roots, if planted too deep those roots cant function.

morz8 has a good point.
I would take it out of the ground and take a good look at the root ball. It can be rootbound and if surrounded by loose soil the moisture in the ground can't transfer to the root ball as fast as it is being transpired by the leaves. That's what I would do.
You, however, may want to pack the loose soil around the rootball and water it very slow and long, rather than pulling it up for a look see.
Check the trunk also for any damage near the soil line.
Mike

If the root ball got dry, no amount of watering will penetrate it. When you have it out of the ground checking the root ball, soak the root ball in water until it gets nice and heavy. It may take half an hour to an hour.

Rhododendron guru Harold Greer of Greer Gardens in Eugene, Oregon, wrote:

"Quite often a plant will get completely dry and then no matter how much water you apply, the rootball will just keep shedding it. The top of the soil may seem wet, and the soil around the plant may even be very wet, but the actual rootball of the plant is bone dry. This is especially true for newly planted rhododendrons, and it is the major reason for failure, or at least less than great success with that new plant. It is hard to believe that a plant can be within mere inches of a sprinkler that has been running for hours and still be dry, yet it can be SO TRUE!"

Here is a link that might be useful: How to care for rhododendrons

Thanks, guys! I will dig it up this afternoon and take a look at the root ball. I always loosen up the root ball, even cutting the roots if it is pot-bound. I don't remember the roots on this particular plant, though, so maybe I didn't do it quite enough.

I was out of town for a week, but we had lots of rain during that time. It was when I arrived back home that I noticed it. In early spring, right after planting, it stayed unseasonably cool, but with little rain. Do you think it could be a delayed reaction from that time? It looked healthy up until about a week ago.

I'll let you know what I find, and even take a pic if it looks interesting. I appreciate your expertise!
Karen

Well I dug it up and did notice the root ball was a bit dry. I had taken scissors to the outside of the clump when I originally planted it, but had not really loosened it up further in, so I did that, gave it a good couple-hour soaking and replanted. That was Friday morning--it got watered in well at the time, and then another 3 inches of rain on it today. Still looks pitiful, but I assume it will be awhile before it bounces back, if it will. Anything else I should do besides wait?
Thanks,
Karen