Check out [2] for an example of one.
Perhaps we should be pressuring companies like Dropbox (whom was implicated as "coming soon" in the documents leaked) to implement such a system.
[1] http://en.wikipedia.org/wiki/Warrant_canary
[2] http://www.rsync.net/resources/notices/canary.txt
Edit: Now thinking about it, Dropbox was probably already hit with a request. Maybe we should focus on other services that may be of future interest to the government?
Or perhaps that's the charitable interpretation.
However, when you saw blips from people paying $99 for your packet of case law on 4th amendment protections with respect to stewardship of online data, you'd have the start of a signal. That is, for all of about ten minutes until you couldn't find a hosting provider or payment processor on Earth who would return your phone calls.
Not that it would matter because a "Chinese PLA hacker nuked your box, your bank account, and your house from orbit."
[Insert relevant quotes regarding government requests on named individuals from Eric Schmidt here.]