There's a new event out for Black and White for Darkrai. Nothing about it is known except the event will start at May 9th, 2012.
Besides that, there is no way to get Darkrai in-game.
Not without trading or if they release another event later on.
*Well you could Poke-Transfer the Darkrai into Pokemon White/Black
*You can get it from a trade from the GTS or from a friend
*Finally, there could be an event in the future
This is for Gen.5 OK
Hope this helps =P
Of course. You needed to get the Member Card, which was an event a while back. You could then go to Newmoon Island and capture Darkrai there. There have also been a slew of other events that have happened in the last few years.