Notes Juniper (via Fire):
The majority of malicious applications target communications, location, or other personal identifying information. Of the known Android malware samples, 55 percent, acts in one way or another as spyware. The other major type of attack, which make up 44%, are SMS Trojans, which send SMS messages to premium rate numbers owned by the attacker in the background of a legitimate application, without the person's knowledge. Once these messages are sent, the money is not recoverable, and the owners of these premium rate numbers are generally anonymous.
Unlike the iOS App Store, which has a review process and tight screening, the open nature of the Android Marketplace allows for widespread malware to be uploaded.