Some apps have password options built into them like iSSH
The built-in security doesn't really address this. You can stop apps from being deleted, but not actual access.
For example. The iPad 2 I got is for the kids mainly. Still I'd ideally leave some apps I use up there: iTeleport, Mobile RSS, etc.. They are little though and especially for the youngest. Wrong click can cause problems depending on the program. For me I just don't put any of my stuff up there. This or profiles would be nice.