Mixing / Production

Why is it that reverb is so much more audible on good headphones than good speakers? Is it to do with the ambient noise or reflections in the room?

I produce electronic dance music. Much of the time I work with headphones on, to help me focus on detailed sounds in repetitive loops. However, I often notice that when I switch to listening on speakers (to check the actual mix and so on) the track seems a bit faster. What could be the cause?

It's often necessary to work on headphones in the home studio, even when mixing. So what headphones should you choose, and how do you go about getting the best results?