The Bouncer provided automated scanning of the Android Market to seek out and potentially malicious software with-out disrupting our market experience or requiring developers to suffer through an application process. Bouncer looks though existing apps, new apps and developer accounts. Once an app is uploaded Bouncer gets to work to make sure they are really 21. He check for known trojans, spyware and malware and any app behaviors that seem suspicious. They actually run the app in Google’s cloud infrastructure and simulate how it will run on an Android device. It helps Google and Bouncer catch any red flags that might get presented. On the developer account side, they check new accounts to help prevent repeat offenders from creating another account to try to upload something destructive.
There is no 100% sure-fire way to prevent things from being unleashed. Google is at least trying to keep as much of what is malicious from landing in the official market. Now if you pick up third-party apps from various other markets or side-load other apps, that is your choice. Rest assured though Google is doing what it can to help. To read up a little deeper about Android and Security, head over to the googlemobile.blogspot.
Via Google Blog, Thanks Bill