It's not clear what you're looking for. Are you trying to physically "lock" (i.e. attach) an iPad to a stationary object to prevent its theft or are you looking for a software "lock" to prevent access to particular apps, documents, email, etc?
I was thinking of a special accessory that turns your iPad into a high tech bicycle lock; a combination pin, facial recognition, and voice analyzer. You could really impress your friends;
and would probably result the first crime where the thief hacks through the bike to steal the lock.