There is a new CryptoLocker variant that is using Dropbox and Google Docs as the delivery method.
This one begins as a very simple plain text email pretending to be an email delivered fax. In an effort to evade filters or at least make blocking these a bit harder, the cyber thief has been utilizing DropBox links to give to potential victims. Much like many campaigns in the past, other virus campaigns have attempted to utilize legitmate, especially free, services to hide their malware. Google Docs was a favorite of spammers to peddle their pharma campaigns, but Google was usually pretty quick to clean those up. In this instance it would appear that DropBox does not scan their stored files for malware and CryptoLocker is taking full advantage of this.
Here is a good article about the problem: http://www.proofpoint.com/threatinsight/posts/attackers-making-malware-delivery-more-secure.php
If you do get infected with CryptoLocker, the only way to get your data back is to restore from backup.
Remember that if you receive an email from an unknown source, don’t open it or click any links.