Now if someone wants to build a client that checks for updates by pinging some central server, whatever, I might even use it. But it's important that anyone be on the same footing regarding building clients, no one gets to use to chain to send messages.
You could use the chain to send messages by constructing transactions that sent money to yourself that others could watch for.
The client developer owns address XYZ (hardcoded)
The client knows any transaction from address XYZ will be informational.
The developer sends a transaction from XYZ to any other address that he owns.
The amount encodes a certain amount of state - so a 1 BTC transaction might be 'there is a new version'
The developer can then send the money back to the XYZ account to resend the next message with.