If you follow the underwebs, you'd know that it is already pretty easy to defeat a jailbroken iPhone's passcode lock, opening up the contents to full review, assuming its not been remotely wiped. Its a bit more of a challenge to do this with a non-jailbroken iPhone, although there are some apps floating around out there that can do it with a little bit more risk.
No word on whether or not this method works on iPads but I'd bet good money that it will.
Short version...don't keep anything on your iPhone/iPad that you wouldn't want to become public information. And if you do lose your device, change ALL your passwords to email, apps, iTunes, etc immediately.