Generation of unlabeled acyclic digraphs
If you are looking for an implementation, Sage knows how to generate general digraphs up to isomorphism

sage: len(list(digraphs(4)))
218


You can then plug in a "test" method if you just want the acyclic ones :

sage: len(list(digraphs(4, property = lambda g:g.is_directed_acyclic())))
31


It returns 302 digraphs on 5 vertices, and 5984 on 6 vertices. But I am a bit scared to try larger values :-)

By the way, giving those values to the OEIS returns the expected sequence :

http://oeis.org/search?q=1,2,6,31,302,5984&amp;language=english&amp;go=Search

Which also happens to contain several references that may suit your taste :-)

Nathann