Many answers are going to be workarounds. As web apps improve and solve these issues the workarounds break. Even official answers may change drastically over short periods of time with some web apps.
How do we handle this sort of change?
I know on many Stack Exchange apps, the best answers are timeless, but it seems hard to give timeless answers for web apps.
Can moderators mark questions as "unanswered" if they notice a solution that previously worked no longer does?
The best people to spot this sort of thing will be those that encounter a problem that isn't solved by the accepted answer.
In that case there's a number of things they can do:
Anything else would put too much onus on the moderators to be constantly reviewing questions when they have (presumably) other things to do - like earn a living.
I guess users with enough reputation could mark the questions as being no longer relevant and pointing out to any potential new answers to the problem.
Something like this (at the top of the question):
This question is no longer relevant as of 2010/07/20, see below for related questions:
If there's a question for which the answer no longer applies, update the answer! That's the reason we have edit/revision capability. The site is supposed to represent the current answer to the question (i.e. multitasking on iPhone used to be impossible, so those answers would now change with the recent release of iOS 4.0).
We can handle this the exact same way that Stack Overflow or Wikipedia does. A dedicated community with editing powers can handle changes faster than they can even happen.
