According to Salon:

In a follow-up interview, we asked Corallo if he could provide us a copy of the letter -- and he says there was a letter -- that Luskin says he received from Fitzgerald. He declined to do so, saying it has been the Rove team's long-standing policy not to release written communication from the special prosecutor. He insisted, however, that the letter says nothing at all about Rove's continuing cooperation with Fitzgerald's investigation. "There is nothing, there are no conditions, there were no discussions of any conditions or deals or anything of the sort," Corallo said. He said that Fitzgerald's decision was based solely on "Mr. Fitzgerald's coming to the proper conclusion" about Rove's legal culpability, or lack thereof.

http://salon.com/politics/war_room/