The Bitcoin protocol lists an Alert message that in the Standard Client is displayed to all users. How does the Standard Client decide which messages to forward and display, and which to ignore? Does it respond only to predefined signatures of the developers, or can anyone send their own alert messages to the Network?
As described in the Alerts article on the Bitcoin wiki:
Only alerts that are signed by a specific ECDSA public key are considered valid. A copy of the private key is held by at least Satoshi, Gavin, and theymos.
All alerts that have not expired are relayed but the client displays only the highest priority alert that hasn't yet expired and wasn't cancelled, I believe.