TechJapan has hand-translated an interview with Fumio Takizawa, the Section Manager of FeliCa Networks PR Department. FeliCa is the hardware that enables NTT DoCoMo's new service that integrates electronic cash chips into a new line of mobile phones, enabling users to pay for small transactions simply by waving their phone in front of enabled sensors. Among other things, one of the distinctions made is how FeliCa itself is a fairly simple technology, and that additional security features must be implemented by the phone vendors themselves.
Although it has been called insecure, rather than the problem being FeliCa itself, I think the problem revolves around prepaid electronic money services, for example - what would happen if you dropped it, or images of accidentally using money come to mind. But since Edy can only store up to 50,000 yen, the risk is about the same as if you were to lose your wallet.
DoCoMo i-Mode FeliCa, Small Purchases With a Wave of Your Phone [Gizmodo]