For those of us (like me) in the UK, this is a very controversial topic.
As a amateur photographer in the UK, you have to know that there are some areas where, if you try and take photographs, you will be stopped by the Police, or what we now call PCSO's (they used to be called "Special Constables" - basically,volunteers with an amount of Police training).
I live in Nottingham (in the 'middle' of the UK) and although there has never been any form of terrorist attack there, the local Police take their responsibilities very seriously indeed.
If, for example, you were to use a dSLR in Nottingham's Market Square, and a passing Policeman noticed you, it would be likely that they would ask you what you were doing, who you were, and move you along. Also, anyone who's ever taken photographs near buildings with 'private' Security guards will be well used to them coming over and challenging you.
In addition to the advertising poster quoted earlier in this thread, here in the UK there are others, and there's also encouragement for members of the public to phone the local Police and report anyone they perceive to be behaving strangely (personal opinion on that of course would vary wildly!)
Some opinion in the UK says we have 'suddenly' found ourselves in a surveillance society - apparently the UK now has more CCTV cameras than anywhere else, in addition to encouragement of the public to report anything they deem as suspicious.
Is this an infringement of "civil liberties"? Absolutely. But that has to be balanced, I guess, by the common, greater, good.
For photographers in the UK (and elsewhere, of course) by all means know your rights, and - politely - stick to them. But don't be surprised if you're asked to stop taking pictures and move on. I know I have been, several times, even when I know that - to the letter of the law - I'm in the right.
Like many, I might not agree with this, and it can be - sometimes - overly heavy-handed.
But, whilst we might not agree with these laws, they are there for our mutual protection, and there's really nothing "we", as photographers, can argue against that.