What is the northern limit of palm trees in the eastern US?

I would say that the northern limit of palms on the east coast of North America would be coastal Rhode Island (borderline plant zone 7a/7b) or Nantucket (solid plant zone 7b) (RI is farther north, but Nantucket is technically farther up the coast). It is generally accepted that both the Dwarf Palmetto (Sabal minor, a non-trunking palm similar and related to S. palmetto) and the Needle Palm (Rhapidophylum hystrix) can survive AND reproduce in areas as far north as the New York area (coastal southern Connecticut). Because summer heat is an important factor in the survival of these specimens, they are easier to grow in southern CT than RI and the MA islands; the climates of the latter two are too maritime in the summer months to produce substantial growth, but, it still is very possible. Sabal palmetto can be grown in zone 7 areas with success if protected from the cold and sited appropriately on a south-facing foundation. But, in terms of a northern limit of palms, they would not be included because they would not survive "naturally." Windmill palms are a bit hardier than Sabal palmettos, but still need to be protected in northern zone 7 climates. Windmill palms (Trachycarpus fortunei) do well in areas with heavy maritime influence and may very well be better adapted to places like Nantucket than either the Needle Palm or the Dwarf Palmetto. More gardeners in the areas around New York ought to try some of these palm varieties.