My isp just sent this warning and since this virus looks like it's coming from the FBI, it might be tempting to open:

Dear Nauticom Customer,

As a courtesy to you, we are advising that you be aware of Internet
Virus/Email Worm, W32.Sober.K@mm. W32.Sober.K@mm is a mass-mailing worm
that sends itself to email addresses gathered from a compromised computer.
The FBI has issued a warning about the W32.Sober.K@mm virus, as it sometimes
poses as a message from the federal law enforcement agency.

The From Address is spoofed. Possible From Addresses:
Officer@FBI.gov
Admin@FBI.gov
Web@FBI.gov
FBI@FBI.gov
Service
Webmaster
Register
Security@microsoft.com

Possible Subjects:
Mail_delivery_failed
Alert! New Sober Worm!
Attachment: No Virus found
Mail-Scanner: No Virus detected
AntiVirus: Found to be clean
You visit illegal websites

Possible Message Text:

Dear Sir/Madam,

we have logged your IP-address on more than 40 illegal Websites.

Important: Please answer our questions!
The list of questions are attached.

Yours faithfully,
M. John Stellford

Federal Bureau of Investigation -FBI-
935 Pennsylvania Avenue, NW, Room 2130 Washington, DC 20535
(202) 324-3000