Some iPhone users have recently found strange messages appearing on their phones, only accompanied by a black background and a big green dismiss button.
These messages are actually a special type of text message, known as a flash or class 0 sms. On most phones it appears as a text message, but does not save to the phone or SIM. On the iPhone however, it pops up a full screen message with dismiss button.
Normally only network operators send this kind of message (for example to display remaining pay-as-you-go balance), but for a while on the O2 UK network one could send a class 0 message by prefixing the sms with *go. I found out about them by one of my friends sending me hilarious error messages through this loophole. Needless to say, O2 have stopped this now.
The iPhone appears to be in the minority of phones not displaying the source of the message, with the close runner up being Windows Mobile phones showing the text as a Network Message.