I mainly use my iPad for business purposes but also use it at home for leisure. Sometimes it gets used by family members, and visiting friends who want to surf the web.
The problem with this is that sensitive business data could be accessed/accidentally deleted by family/friends. So I really appreciate it when developers include a passcode lock on their Apps. However, not all developers do this and it can makes the difference between me adding their business App on my device or not. The ideal solution would be for Apple to build it into the operating system, here's hoping.
My suggestion would be to allocate it to a folder - so when you try to open a folder of Apps, it has a passcode lock on it.
I run a CRM / project management system with sensitive client data on it, a journal app and access email. I also have some Apps that my 10 month old daughter uses and then some party games that are great in small groups. If Apple could implement a passcode system on a folder, I could put all my business/sensitive data apps in a protected folder and the rest of the apps can be accessed by friends and family. This way my data's safety isn't reliant on developers enabling passcodes but instead responsibility is my my hands to set a passcode to a folder.
There could be many other scenarios - another example: at a client meeting, you could pass them the iPad to look through portfolio without worrying about them seeing sensitive data about prices/supplier costs. Or maybe you ordered that special surprise birthday gift for your partner and want to ensure she doesn't accidently open Mail and have her surprise ruined. I'm sure you can think of others.
I suggested this in a support forum of one of the Apps I use and this one guy (his title included 'information solutions') told me to put a passcode on the iPad itself, which of course everyone knows and is fine if you want to lock the whole device - but what if family want to surf the net? Do I say "no, you may accidentally wipe my data"?. His answer was to buy a separate iPad for each person in the family/friends - really? Wow, how lame is that for a solution? What if someone's iPad is charging and they need to use another?
I think Apple could really help it's user base by making this feature (or a better solution if they have one - hey I don't mind as long as it works) available in the next version of iOS and as a bit of a geeky developer myself, I can't see it being all that difficult - not for Apple anyway.
If you agree help others out by sending Apple a quick feature request asking for passcode locking folders for apps.