I am not aware of any cases of warrant canaries being tested by the courts, but the general principle has been effective in the past in the UK.
When cars were first introduced, many towns across the UK viewed them as an opportunity to make money by fining the rich by setting obscenely low speed limits (think <20mph) with extraordinarily high fines. In response to this, the AA was formed to warn motorists of speed traps down the road.
This started to cut into profits, so the the government retaliated by charging and convicting an AA guy with "obstruction of justice". AA agents were therefore not allowed to inform motorists of speed traps.
The AA responded by changing their protocol. They would always salute passing motorists unless there was something wrong (aka, unless there was a speed trap). The absence of a salute indicated a speed trap, and the law could not force the AA to salute.
I'm not sure if this was ever challenged in court, but they were able to keep it up for several decades so it was never successfully challenged in court at least.
I believe that this remained the habit of motorcyclists until relatively recently and it has since been tested in court and declared not allow (reference vague memories of newspaper articles).
Do motorcyclists signal to others except when there is a speed trap, or do they signal to others when there is a speed trap? The later is common in the states (cars flashing their headlights), but I haven't heard of the former and enforcing a ban on it seems completely impractical at the very least.
And what kind of free society can force citizens to salute? "The government won't tolerate warrant canaries" makes intuitive sense because we have grown used to the courts throwing out all sensibility whenever there are computers involved, but the idea of the government compelling civilians to salute "in meatspace" seems blatantly beyond the pale.
We tap our helmets to warn of cops. We wave (or nod in countries that drive on the left) just to say hi. Absence of a signal would be useless on a bike as we're generally pretty bad about giving the signals we intend to give, let alone not giving the ones we don't intend to. It would be chaos.
When cars were first introduced, many towns across the UK viewed them as an opportunity to make money by fining the rich by setting obscenely low speed limits (think <20mph) with extraordinarily high fines. In response to this, the AA was formed to warn motorists of speed traps down the road.
This started to cut into profits, so the the government retaliated by charging and convicting an AA guy with "obstruction of justice". AA agents were therefore not allowed to inform motorists of speed traps.
The AA responded by changing their protocol. They would always salute passing motorists unless there was something wrong (aka, unless there was a speed trap). The absence of a salute indicated a speed trap, and the law could not force the AA to salute.
I'm not sure if this was ever challenged in court, but they were able to keep it up for several decades so it was never successfully challenged in court at least.
Edit: The AA could be considered sort of similar to the american AAA ("triple-A"). http://en.wikipedia.org/wiki/The_Automobile_Association http://www.theaa.com/aboutaa/history.html#tabview=tab1