My Dad just got an iPod touch and raised an interesting question. Are there security risks for an iOS device? He was planning on storing sensitive personal information on it, but wasn't sure if it was completely safe. Is it even possible to hack into an iOS device and retrieve information?
Currently there aren't any vulnerabilities that can be used remotely to access data like this that I know of.
If someone had physical access to the device, then they could access any data on it, as the disk isn't encrypted. Of course, the high retail value etc would mean most thieves would likely just wipe and sell it, rather than try to extract data.
In short, right now there aren't any active exploits that allow remote data access (though there have been ones in the past involving tiffs and pdfs that may have allowed this).
If the device was stolen, you are out of luck though, even with a passcode lock.
It's possible to set up two things to mitigate this:
Keep in mind that new security vulnerabilities may or may not be published right as they are discovered--It all depends on who finds them and if Apple responds quickly to them. This means that the answer to your question may be different depending on whether or not there is a publicly known security threat that has not been addressed by Apple.
With that said, and with what we collectively know today, the only security risks are tied to you actually giving a third party physical access to your phone. The security issue around that web page that would gain access to your phone has long been addressed by Apple. At the moment, I would consider my MacBook to me more prone to hacking than my iPhone.
I agree with @Rich Bradshaw in that setting a passcode and enabling Find My iPhone for its remote wipe feature are must-haves. Hope this helps.