Answer:
   
   The concept of G-d hiding Himself, has a long tradition in Jewish
   thought. Basically, the idea revolves around the difference between
   obvious acts of G-d (i.e., things that violate the rules of nature)
   and subtle acts of G-d (things that are directed by G-d, but follow
   all the rules of nature).
   
   As examples, consider the Jews saved from Egypt with the sea being
   split, and compare it to the way the Jews are saved from the Persian
   empire in the story of Esther. Jewish commentaries point out that
   G-d's name does not appear in the book of Esther, and yet it is
   presented as a miraculous event. The Talmud points to Deuteronomy
   31:18 as a general description of this phenomena. There, G-d says "I
   will hide my face".
   
   With that as a background, the verse in Habakuk can be understood in a
   number of ways. Rashi--one of the foremost jewish biblical
   scholars--sees the verse as referring to the change that took place
   with the revelation at Sinai, for until that time, G-d was hidden from
   the world.