There is the tweak AppLocker 2 ($.99, USD). It's fairly robust and will allow you to lock apps or folders (or both). The description reads that it's for iOS 5. I had it on my iPhone running 5.0.1 but have not tried to run it on either of my iDevices running 5.1.1. I took it off because it's just me and I really didn't need all that protection. But, it worked fine when I had it.