Phillip Oliver blogged his visit to the Dumbarton Oaks in Washington D.C.
In a year or two of reading garden blogs, I've never heard of it. Apparently, it's not very well known out there either.
Out my way, the big estate garden is Filoli, and everyone knows about it. I blogged my visit last spring here.
ADDED: And then he goes to Hillwood, which has a pet cemetery, people! Link.