I think the form helpers in Rails are handy shortcuts, but I don't know if I'd call them abstractions. Abstractions are tools for generalizing, but Rails helpers only produce HTML output. Only if you could re-use the same helpers to produce a variety of different types of markup would I call them an abstraction (and that would probably make it necessary to make the abstraction less transparent).

A better example of a true abstraction in Rails is ActiveRecord, which I think you'll agree is bit less transparent.