Invitation Card.zip is the name of the malicious attachment, and it is being identified as W32.Ackantta.B@mm, which was first discovered in an e-card virus attack in February. W32.Ackantta.B@mm is a mass-mailing worm that gathers email addresses from the compromised computer and spreads by copying itself to removable drives and shared folders.
Here is what the message looks like in an inbox:

From: invitations@twitter.com
Subject: Your friend invited you to twitter!
As Twitter continues to gain popularity among social networking users, people are regularly receiving invitations and email updates from fellow users. We expect that spammers will continue to use Twitter and other popular social networks as bait in their attacks.