The best defense is a smart, aware user. Trojans will always get through. Telling people they're 100% safe because all apps are screened builds a false sense of security in the best guard there is - you.
Is it Droid or that ever-looming, never materializing Microsoft phone, that uses voice recognition to identify keywords in order to target ads to the user of the phone? Does this not strike you as excessive? It certainly strikes me as excessive.