@ Tom 38, Presumption of innocense?
"Precisely what law has Birgitta Jónsdóttir broken? Nice to see you assume that she is a brazen criminal, since the US authorities are doing some fishing."
Uhm dude, hate to break it to you...
First this isn't a fishing expedition. Its a very limited search on a few individuals who have already made their identities public and there is a legitimate tie between them and Wikileaks.
You seem confused by the concept of 'presumed innocent until proven guilty'.
It appears that you are the one making an assumption that Birgitta Jonsdottir has been accused of breaking a law since her Twitter account is one of those under investigation. The fact that they are investigating means that they don't know the answer and are looking for clues and more information to determine who did what and when.
But I digress. The point I wanted to make was that the presumption of innocence deals with the defendant during trial. That is that the defendant is presumed innocent until the prosecution can show that there is proof beyond a reasonable doubt that the defendant is guilty.
You don't charge anyone with a crime until you have concluded your investigation and this is why they are asking Twitter to honor their subpoena request.
And using your hypothetical example... if your ex-GF has credible evidence of rape or enough to warrant a police investigation, then the answer is yes. If there is something in your Facebook page, or viewing history that is relevant to the rape investigation then the police do have the right to search your internet surfing habits.