The greatest current risk for iPhone owners is not viruses or malicious web pages, it is the danger that the phone might fall into someone else's hands. Although iPhones do offer elaborate security mechanisms, these mechanisms won't stand up to an imaginative hacker.[h-online.com]

Also:

The H's in-depth article on iPhone security, iOpener, vividly demonstrates that you can't turn your iPhone into a tamper-proof data vault, but the following measures will make life considerably more difficult for thieves.[h-online.com]