What to do after you lost restriction code on the iPad? 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.
What amaksoud said is true. I have the original iPad on 4.3.1 and after trying other advice decided to "read" the com.apple.springboard.plist. 

Here's how to do it and no need to jailbreak. 

Download iFunBox; it's free and works for OS X and Windows. Choose Raw File System and navigate to var/mobile/Library/Preferences. Select the file named com.apple.springboard.plist and choose copy to Mac (in my case). Copy the file to your preferred location. 

Then in Finder you select the com.apple.springboard.plist (that you just copied to your Mac) and look for &lt;key&gt;SBParentalControlsPIN&lt;/key&gt;. The 4 numbers located between the tag &lt;string&gt; &lt;/string&gt; are your Restriction passcode.

For example &lt;string&gt;1234&lt;/string&gt; means your passcode is 1234.

Hope this helps.