There have been a number of interesting articles about the sign outside Aldwych tube station, e.g. amateur photographer article. The sign reads "Due to their combination of high quality sensor and high resolution, digital SLR cameras are unfortunately not permitted inside the station." which is a strange thing to say.
As many people have commented and tweeted, it is idiotic. It also inconsistent with other statements made that said the ban was professional camera equipment, tripods, and for safety reasons. What is even more daft is that they were allowing high resolution non-DSLR camers and SLR film cameras.
From a safety or slowing-down-the-tour viewpoint a modern DSLR will be able to take pictures much more quickly than most compacts or phone cameras, and cause much less disruption.
What gets me is that this is a typical trend for anyone doing any sort of administration and trying to get people to do what they want (or not do it). Rather than say "we do not want you taking good pictures that would undermine our sales of photographs" which is (what many people guess) the reason for it all - they make up some plausible excuse such as safety or slowing down the tour, etc.
The big problems with such made up excuses, rather than saying what you mean, is that the silly excuse can usually be challenged as stupid or bypassed (e.g. using a film SLRs), defeating the whole object. Indeed, there are a whole type of person (like me) that would go out of our way to technically comply with the silly excuse whilst deliberately defeating the presumed actual intention (by taking a film SLR, or a compact camera on a tripod, etc.).
One problem is that the real reason is usually one that people do not want to own up to. It is possibly not justifiable or objective. People like to invent an excuse that is somehow objective and has some justification instead.
I recall one occasion when I worked for STL and they said that they wanted clean/tidy desks. I am not sure why, or how they would justify it. I expect it was just to look tidy, but that was never clear as the excuse was for security reasons. Of course, as a made-up excuse it was hard for them to elaborate. They did not want to provide locked cabinets for everything (that would cost money). They said that they did not want anyone to be able to casually see anything confidential. They had said that security personnel could confiscate anything confidential that was left visible on desks, so clearly security staff were not the concern. The result was not tidier desks but a standard front page on all printouts saying "non confidential cover page" and the document name. This addressed the stated concern perfectly as nobody would casually see anything but non-confidential cover pages, and the desks could stay as a "I am in the middle of working on something and do not want to waste 10 minutes in the morning getting it all back where I want it", AKA untidy, state.
I suppose it is that people like a sense of self importance in jobs like this. They should take up herding cats rather than try and direct pedants with made up excuses.