the droid market place has been an open environment and recently malicous apps have been found on the market place and downloaded by millions

http://androidcommunity.com/android-users-hit-with-data-theft-by-malicious-app-20100729/

http://www.phonearena.com/htmls/Malicious-banking-app-found-in-the-Android-Marketplace-article-a_8744.html