While American teens are routinely charged for defacing government websites or hacking, it takes a daring (or foolhardy) hacker to pretend to be the FBI in order to slap more viruses onto unsuspecting users' computers.
The real FBI said;
"These e-mails did not come from the FBI. Recipients of this or similar solicitations should know that the FBI does not engage in the practice of sending unsolicited e-mails to the public in this manner.
Opening e-mail attachments from an unknown sender is a risky and dangerous endeavour as such attachments frequently contain viruses that can infect the recipient's computer. The FBI strongly encourages computer users not to open such attachments".
So, as normal, update your virus suites and whether you're using Outlook or online mail, delete it if it's unfamiliar. We'll keep you posted on whether (or maybe when) someone is caught and charged.