Moments ago, Google unveiled a security service that has been working behind the scenes on Android for a few months now, codenamed Bouncer. The service allows the Android Market to scan apps that have been uploaded, analyze them for trojans or other malware, and then determine if they have tendencies that would lead them to misbehave. Again, this has been going on for months to protect you, Google is just now letting you know about how successful it has been.