Are there families of formal languages known to be truly PAC learnable? I specifically mean language families that admit arbitrarily long strings -- not conjunctions over n bits or decision lists or any other "simple" language contained in {0,1}^n.

I am asking about "automata-theoretic" regular languages as opposed to "logic-theoretic" ones: something like the piecewise testable languages, start-height-zero languages, locally testable languages, that sort of thing. The relevant complexity parameter n is the size of the minimal accepting DFA. So, succinctly put: is there an interesting family of n-state DFAs that is known to be efficiently PAC learnable?
There is a recent result on the polynomial pac-learnability of semilinear sets at LICS 2010: Parikh Images of Regular Languages: Complexity and Applications. I guess this is not what you are looking for.

You should also have a look to the paper of Clark and Thollard: PAC-learnability of Probabilistic Deterministic Finite State Automata