There's always a solution, and most of you won't like it: jailbreaking. SBSettings (the #1 reason to jailbreak in the first place) gives you the option to hide icons from your home screen. So if you plan on leaving your iPad out of your reach for a time and you want your mail hidden, you could always hide the application itself and restore it later. Just my two cents, didn't see this suggested anywhere. Only "downside" is jailbreaking, which is ridiculously easy to do (click of a button with Spirit) and relatively easy to get rid of without Apple knowing. Also, it's a bit of a pain (repetitive) to always hide and unhide the icon, but hey, what other choice do we have right now? Hope Apple will rectify this somewhat in 4.0, or at least allow Mail to be a "restricted" app inside of Settings > General > Restrictions.