I didn't like the idea of an in-the-software alert. I was convinced that it wouldn't be a problem, but I do think that having many competing alert systems will yield the best one.
There is no reason to think Satoshi will have the fastest reaction time or the best ability to determine that there is a problem. Maybe he will, but offering alternatives is a good idea imo.
You could do one via DNS. Specify a TXT record and have people poll it. If it contains anything, you triggered an alert. This is DoS-resistant because the DNS uses a lot of caching and the responses are small.
The record itself could contain a URL to a signed message with more info. Possibly DNSSEC could be used.