How can I lock Mail/Contact/Calendars in an iPad?
Without jailbreaking, I believe your options are the use of apps that provide this kind of lock functionality built-in, unless Apple decide to implement some kind of multiple user support in future iOS versions for devices like the iPad. 

I've previously come across Secure Gmail or Secure Webmail (there may well be others) which provide passcode protection schemes when starting and, judging by the screenshots, appear to be frontends to the relevant webmail pages (as opposed to a native application like the Apple Mail app). I've not used either, but they may be of use to you.