iPhone 3.1: Remotely Set a Passcode Lock via MobileMe
With Find My iPhone, you can help protect the information on a missing iPhone by remotely assigning a four-digit passcode lock. This is especially useful if you've never set a passcode before or if you want to set a new one.
This is a nice half-way ground between leaving it wide open, or having to wipe it clean, if you can't find it but aren't sure you've personally lost it.
And, again, it shows MobileMe is all about the incremental update. Very incremental...