iPhone "Slide for Emergency" and "Activated" error messages

I'm not sure what I keep doing, but every once in a while when I turn on my iPhone 3G (after turning it off completely, i.e., a restart/reboot) I see a couple of different weird "error" messages. The first error message says, "Slide for emergency", and looks like this:

(As I look at it now I see there was an "information" icon on the screen, but I didn't see it during my earlier panic attacks.)

"iPhone is activated", dismiss (error) message

The other message, which in my mind is really an error message, says "iPhone is activated, dismiss", and looks like this:

I'm not sure why I would ever get this message from my iPhone any more, as I bought it over a year ago, so that's I call it an error message.

"Fixing" the problems

Regardless of the cause of these messages after a reboot, I've found that by turning the iPhone off completely (essentially restarting it again), and then turning it back on these messages went away. These messages are a little disconcerting, but at least they go away easily.