On Cryptogon is an article about the CIA and U.S. State Department targeting of Wikileaks personnel for surveillance. Ultimately from this article and also one on Harper's Magazine and True/Slant you get the picture that the Pentagon, the CIA and State Department all want to get rid of this troublesome site. Troublesome because it provides a forum for whistleblowers to expose the secretive actions of governments and corporations today. More specifically however, the recent aggressive surveillance might have something to do with one of their twitter postings:
# WikiLeaks to reveal Pentagon murder-coverup at US National Press Club, Apr 5, 9am; contact email@example.com
That probably is what Wikileaks is refering to when they say this on their twitter page:
If anything happens to us, you know why: it is our Apr 5 film. And you know who is responsible.
UPDATE: According to the Daily Mail, the April 5 US National Press Club presentation by Wikileaks is "its plan to make public unencrypted footage of an air strike in Afghanistan on May 7 last year that killed 97 civilians."