There are a couple of good posts today on “Show Don’t Tell”, that age-old mantra cited in just about every writing class world-wide.
I’ve just started reading The Passage by Justin Cronin and this starts with an entire chapter of exposition. The audacity! But Cronin does it very well. He litters the exposition with little details that highlight character to the extent that by the end of the chapter you really feel for these characters. It’s a brilliant bit of writing, but then it’s clear he knows what he’s doing.
As writer Adam Christopher says in the comments of Mark’s blog post

I think rules are essential for learning the craft, but once you’re there, you can do what you like. It’s like driving. You don’t become a stunt or racing driver without learning to drive first. That’s the basic foundation. Once you’ve got that, you can do what you like.

I think this is true. It’s very easy to say that writing is simple. In essence it is: you put one word after the other. The real skill comes in selecting which word and in which order, and I think it’s fair to say this is a skill no-one has ever truly mastered.
So there are times when ‘tell’ is better than ‘show’, but you have to know you’re doing it. It’s very easy for your story to go flat when you tell. There are a million and one different ways of writing any scene. What you have to ask yourself is “Is this the best way to write this scene?”
The mantra exists for a reason. It’s easy to fall foul of it; very easy, even for pros. If someone reading your work mentions it, there’s obviously something there that has thrown them out of the story, and maybe there’s a good case for more dynamicism. It’s easy to put up a spirited defence that the exposition serves a purpose, but if people are noticing it, there’s a good chance it’s not working.
And I guess that goes for all the rules. They are there to be broken, but only if the need exists and it will enhance the story