Because there are currently no known remote attacks against iPads, the biggest security risk is physically losing the device. Thus, the first step is to make sure your tablet's data is safe in case it's lost or stolen. For that, I'd suggest a combination of encryption and remote wiping.

All iPads ship with powerful hardware encryption built-in, but you need to enable it. The simplest way to do that is to set a passcode on your iPad: As soon as you do, your data will be automatically encrypted. To enable a passcode, go to Settings -> General -> Passcode Lock and then enter a four-digit code twice. If you'd like to be extra-safe, you can turn the Simple Passcode option on that same page off; you can then use longer codes. While you're there, you should also set Require Passcode for no more than 15 minutes and turn Erase Data on. (Technically, the iPad deletes your encryption key, not the actual data, but that's faster and just as effective.)

All modern iOS devices also come with a second layer of encryption, called Data Protection. While the basic encryption enabled by turning on passcodes protects all of the data on the device (including your apps), it can be bypassed by jailbreaking. Data protection encrypts your e-mail messages and their attachments; it can't be broken even if the passcode is stripped by jailbreaking. Data protection is also available for programmers to use in apps, but few take advantage of it. (At this time, there aren't any jailbreaks for the iPad 2, so the basic encryption is still safe; but that probably won't last forever).