One principle to remember is that no form of security is perfect. If someone has your device in their physical protection and has sufficient skill and/or the right tools, they will be able to get your data.
One example - in an article titled "Limitations of Data Protection in iOS 4", Professor Anthony Vance from BYU notes that:
Updated August 6, 2010: Elcomsoft has announced that its iPhone
Password Breaker tool can recover iPhone keychains (probably the
escrow keybag) from password-protected iPhone backups.
While this announcement is dated and relates to iOS backups on the computer and the security in iOS 5 have improved, tools that can be used to extract data has also improved. Further, iOS 5 still has some security flaws, as this article points out - one example is of the security bypass in iPad with Smart Covers.
Bottom line: you can't completely count on device security to ensure the protection your data if the device is in someone else's physical control.