Why not physically lock it???
There are plenty of useful iPad locks on the market today. Some are only suitable for retail and POS, like the Ram Mount solution which you can't take with as it needs to be bolted into something. Others, like Maclocks iPad lock and security case, are perhaps better for consumers. You can use the lock just about anywhere, and it can be easily removed when you're done.
All those software security suggestions are good to maybe keep your info safe, but why not opt for keeping the actual devise safe?? :)