Short of water. Next, need to find out why.

Wow! You think that with all the rain Irene dropped in our area that it should be enough. Maybe too much water? but why the other rhodies around it are ok?

Don't know. You'll have to investigate.

One reason the top can be short of water is root rot.
Other reasons include mechanical damage and insects.

As said, now need to find out why.

Looks like summer root rot to me; a serious risk to rhododendrons in DC and environs. They can survive some kinds of rot, although more often than not the situation is hopeless. I have my own mitigation approach but it's highly specialized and I'd rather someone like Rhodyman chime in. FWIW I know a professional landscaper who refuses to plant rhododendrons in the DC area even if his clients request them. That's a landscaper's approach - someone like Don Hyatt would obviously disagree.

For now just leave it alone and hope we start having more typical fall weather of highs in the mid 70s and lows in the mid 50s with little rainfall. If there's a downspout or leaky gutter near it, fix it - make sure it's not getting water from any extraneous sources.

anyone on the East Coast but especially south of New England would do well to follow Rarefind/Hank Schannen's advice on planting rhododendrons, which is a list of 10 items of which 4 of the ten items are drainage!
It's taken me a while to learn this but I plant all of mine on a slightly mound now. It helps prevent, in a slighly way, these problems from happening.
Out in the PNW with their cool dry summers I've seen them practically planted near the bottoms of swales and they seemed ok; that's not recommended practice there but for the generally tough hybrid varieties, winter wet short of semi-permanent ponding isn't going to kill them because it's too cold for rot to set in.

arrrghhh getting too tired here, speaking of rain spent 2 hours tonight trying to get my cellar pumped out and dry. This might be the rainiest spell I can remember in 20 years of gardening.

I meant a mound might prevent "in the slightest way" such problems from happening. If you get too much warm hurricane rains following the hottest July ever, the plant might just be doomed.

Thanks everyone, I really dont think it's lack of water, it's been raining like crazy here in the DC area, and still all the leaves curled up. I was thinking maybe a root problem, hoping not root rot, but maybe because it's planted too close to the edge of the raised bed maybe the roots are hitting the edge of the bed and thus limiting the growth of the roots and the absorption of water? I was thinking of lifting it and moving it away from the edge if the roots are good, not rot, would now be a good time?

As the others have hinted at (or directly stated), too MUCH water can cause wilting, too. Especially if it's too much water over a long period of time. Water is absorbed only by the tiniest little root hairs on the tips of the roots. Overly moist conditions can cause those root ends to 1) stop the activity of cell division, and no more root hairs will be 'born' and then to 2)eventually rot. Rotted root ends mean that water cannot be absorbed and the plant will wilt. Exactly as if there's not enough water.

Which is the exact problem, at least (as Jean put it) for the top of the plant. Only you can determine if this location has been a bit waterlogged for too long.