Mmm, here's a short list: Franklinia alathamaha and Torryea taxifolia.

Betula Uber

and a few less rare but still rare ones at least in the wild:
Gymnocladus dioicus
Cotinus obovatus

The link has more...

Here is a link that might be useful: Some Rare Trees

Kentucky coffee-tree is very common around here. There's quite a stand behind our house along the creek!

Here's some that are fairly rare:

-Magnolia ashei

-Halesia parviflora

Alabama Snow Wreath Neviusia alabamensis
Thought to be naturally headed for extinction.

Maple-Leaf Oak Quercus acerifolia

Fothergilla thought not a tree, still notable and endangered.

These are the first that come to mind that have not already been mentioned.

Arktrees

On a similar note to Maple leaf Oak, Quercus Arkansana (Arkansas Oak) is another tree listed as 'Vulnerable'

Here is a link that might be useful: wiki

Torreya taxifolia is rarest..also, Litsea aestivalis, Taxus floridana, Salix floridana, Quercus boyntonii, Although a shrub, check out Rhus michauxii, Alnus maritima, Magnolia pyramidata...I could go on..

One of my favorites, rare in most areas, but prolific in their prefered areas, Rhodedendron Maximum. It needs shade though. I beleive our native Rhodie is a tree. I managed to get 3 this past spring. One died from too much sun, the others are in shade and doing good. They have an almost tropical look when in the right conditions.

Eastern North American is kind of oxymoron considering that Mexico is part of North American and you could say that east Mexico is "eastern" NA. Lol.

In that case, Mexican Sugar Maple. Lol.

Well, anywhere there's trees east of the Rockies, pretty much.

Planera aquatica may or may not be very rare in its native range, but it's one you almost never hear about, anywhere. Jacobson says "exceedingly rare" by which he generally means "in commerce" but I think in this case means a bit of both: in commerce and in the world, period. It is not in the UMN plantfinder.

Here's an amusing one I'd never heard of:
http://ecos.fws.gov/speciesProfile/profile/speciesProfile.action?spcode=Q2WX

Florida ziziphus? (Ziziphus celata) Really just a shrub though. You can look at the link below for more trees:

Here is a link that might be useful: http://ecos.fws.gov/tess_public/SpeciesReport.do?groups=Q&listingType=L&mapstatus=1

Silky Camellia is more shrubby than tree. It is by far the showiest of the stewartias when in flower. It was well-known to all of our colonial era botanists, however today it rarely enters into discussions. I can't name one
commercial grower of this plant, can you?
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Stewartia malacodendron fall color:
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Low population, but I'm not sure how Far East it extends exactly is the Yellowwood tree (American Yellowwood, Kentucky Yellowwood). Very beautiful tree.