When accelerating rapidly from a low speed (first few strides) a short stride is best, you actually can ignore a normal stride entirely for the first couple pushes and really just dig in your toes for extra power and really hard pushes. As you get to speed it's more about minimizing friction, and a longer, smoother, speed skater type stride with a good toe flick at the end seems to be more effective.
To a certain extent though you can (and should) get the best of both worlds - a long, powerful stride when at full speed, but the moment the stride is finished you want to be returning that leg back underneath you as soon as possible to get ready to stride again. Reducing that return time while keeping your push long and powerful can really boost your speed.
Finally, being fast in hockey is more about your ability to conserve speed when turning than it is your straight line speed IMO. The guys who seem really fast tend to be conserving more speed in general, so when they really need to get to the puck fast they've already got a good amount of speed and don't have to start accelerating from a virtual standstill. Quick starts and efficient turns are key.
Watch this vid from 8:50 on, really shows how pretty much everyone uses toe starts at first, then transitions into a longer, more powerful stride, but still with an incredibly quick return. Grabner (at the end) especially has a very long and powerful push, but after the pushes returns his foot to its starting position so quickly.
http://www.youtube.com/watch?v=jE8ym9qMICc&feature=related