So far every time I've done it i have just animated an emitter's rate and placed it where i know the objects will collide. It definitely gets the job done, but i cant help but think there is a really cool way out there that will do this in a more efficient way.. ?? ..
I found this tutorial online http://www.thegnomonworkshop.com/tutorials/collision_emission/collision_emission.html but it is really old and im guessing there are probably new better ways to do this.