"The collection published today has not been leaked. Assange said WikiLeaks had been working for the past year to analyse and assess a vast amount of data held at the U.S. national archives before releasing it in a searchable form."
If the materials were properly declassified and held for public browsing by the NARA then what Wikileaks is doing is fine. It would be the same as Google wrapping a much nicer search interface across an old dataset.
If the materials were still classified-but-leaked then I won't go near it until it is declassified. But I can't tell from this side of the window what's going on.
http://leaksource.wordpress.com/2013/04/08/wikileaks-plusd-p...
So it is legal