Thus, rsync.net has a "warrant canary"--they periodically post a cryptographically signed statement that they have not, to date, received any PATRIOT Act warrants or had any searches and seizures. If they stop updating the statement, then you can draw your own conclusions.
The second of these library signs uses the same principle: "The FBI has not been here [watch closely for removal of this sign]."
(Via jwz's blog, where some commenters question whether the recent Washington Post piece by the recipient of a National Security Letter is truthful. Note that the ACLU has a lawsuit going on about this case, which I previously noted back in 2005.)