An app for IOS (Apple) doesn't exist, and will never exist (in the AppStore), because it would break Apple's strict use of APIs and system modification. See, you'd have to replace the boot loader for an app of this type to work properly, otherwise you'd need to store the key unencrypted to boot the device, and then have an app run on startup. There may eventually be a jail broken app (I'm aware of one that requires a password to run an app, but nothing when it comes to full disk encryption like would be needed.
Android shows some promise in this category, but you'll still likely need a rooted device for it to work, as no carrier is going to let an app control the boot loader, again, which is needed for this app to work. (It's how they all work, like TrueCrypt, DriveCrypt, BitLocker, etc - the boot loader looks for the password or token/key, then uses it to unlock the rest.) Since. Phone is also used as an emergency tool, an encryption app would likely need to include a way to dial an emergency number without the password (like the current passcode solution does on IOS), otherwise they could get in hot water. There's always a trade off, as security and ease of use will never go hand in hand.