Yup, I said it. Sometimes it's okay to tell. Now before you start booing and throwing tomatoes at me, let me give you a few examples.
Every once in a while you need a hint into a characters past that will foreshadow upcoming events, but lots of flash backs are a no-no. If you can sum it up in a simple sentence or a line of dialogue, do it! Unless it's for comedic value such as this little gem. (sorry, it wouldn't let me embed it)
Sometimes something important happens and one character needs to tell another character. Please, please, please, don't go and show the story again. A summary is best in this situation. Here's an example of this done well in Rick Riordan's 'The Throne of Fire'
"I gave them the shortest version possible -- why I left London, how the Egyptian Gods had escaped into the world, how I'd discovered my ancestry as a magician. I told them about our fight with Set, the rise of Apophis, and our insane idea to awaken the god Ra."
3. Un-important scenes
It can be easy to get caught up in so much showing that soon you are showing things that are of no consequence. No one needs ten pages on how someone got onto an airplane and found their seat. Unless it furthers the story, telling the reader they got on the plane may be enough.
Are there other times when telling is best?