This is partly true and partly false
The best place to check for clarity, and clear up the lies from the truth, and learn further details about this virus is Snopes itself..
Snopes reports this virus as real.. and goes into much more details about various other email subject headings it can send itself as.. but.. it also clears up a few points as not true..
Although the Postcard virus is real, it isn't a "BIG VIRUS COMING" (it's already been around in multiple forms for a long time now), it will not "burn the whole hard disc" of your computer, CNN didn't classify it as the "worst virus" ever, and it doesn't arrive in messages bearing a subject line of 'Invitation.'
There are a lot of viruses around that do the same thing.. the general rule is don't open something that anyone sends, if the message accompanied with it seems out of their usual character, or there's no message at all.. always check they did send it.. and if they haven't acknowledged that they sent the thing then it'll be a virus