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I guess the title says it all. I've the restriction code long time ago, and when I wanted to do actuall in-app purchase it turned out I cannot recall the password. Worse, now after 10 tries I have to wait 60 minutes before each consecutive tries.
The threads on forums are somehow misleading. Some of them claim the only way is to do fresh install and recover all the stuph manually. Some other people post some not entirely clear solutions involving restore from backup. Closest I got was the iPhoneBackupExtractor tool, which is suppose to read the plain text key code from the iPad backup. It worked to the point of getting hands on com.apple.springboard.plist. Unfortunately the password is not in this file.
Please keep in mind, this is about the "restriction code", not the main lock code. iPad if fully functional and the one feature blocked is in-app purchase.