I've posted this numerous times, but people contrinue to think that the prosecutor will give ample warning to the target. That's just not the case. Targets are typically notified that they are targets a few minutes or hours before they are indicted. There is no reason for any prosecutor to let someone know they are a target until the last possible moment. By telling them they are not currently targets but that they may be witnesses or suspects, the prosecutor is able to coerce the "non-target" into testifying before the GJ. Then, the prosecutor can use that testimony to go after perjury or obstruction of justice if necessary. It makes a nice fallback if he is unable to get enough evidence to go after an indictment on the original crime. If the prosecutor tells the person he is a target up front, you can bet that he will not cooperate.
Also, in this case, it has been stated repeatedly by Rove's attorney, and repeated again today after the testimony, that Rove has not received a target letter.