In other words, if my iOS device is stolen, will the person be able to get my data by pluging it in a PC and use some trick (or by being able to jailbreak my device while it is protected with a passcode lock)?
For the discussion, please assume my iOS device:
In general, no. iOS devices where you have enabled hardware data protection on iOS 4 or later require the correct passcode to be entered. If you haven't enabled a passcode, the data on the phone is easily available to the thief assuming they have either minimal training and/or money.
So, as long as they can't guess the passcode from looking at smudges in the glass or recording your motions before the device is stolen - you are as secure as the passcode you have chosen. Most times when the government needs to bypass the passcode locks, they get a warrant for your home computer and take the key that iTunes uses to unlock the device and use specialized software to send that key.
Once someone is willing to spend thousands of dollars on either specialized hardware and software or thousands of dollars to hire someone trained in iPhone forensics to do the work, the chances of "cracking" into the device are higher, but still not assured if you have chosen a good passcode and not allowed your keys to be stolen by a backup to iCloud or let the thief get to your computer as well.
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.
1) Answer is yes, they may be able to get your data if you
are using a device model prior to iPhone 3GS.
jailbroken the device and didn't secure published security loopholes (google for more details).
didn't set Settings.app->General->Auto-Lock to a time period, and didn't set Settings.app->General->Passcode Lock->Erase Data to On after X number of unsuccessful password attempts.
didn't encrypt your backup (on your Mac or PC that has iTunes). Prior discussion Stop my old iPhone from backing up
2) Another question is will they be able to quickly make sense of the data. Prior to iPhone 3GS, yes; post 3GS, the data on the Flash Memory is hardware encrypted. Each successive (newer than 3GS) devices use encryption algorithms that are more computationally-expensive to decrypt, except perhaps for 3-letter government agencies whom Apple (by law) has to submit their encryption algorithms to.
