When you say leaking, how much 'stuff' is coming out of the pipe? I looked at my battery and the pipe that comes from it is to vent the gasses present during normal battery usage! If fluid is coming out, then I'd say that your battery is either overfilled with acid or it's being 'boiled by a dying/duff regulator
Stick a voltage meter across the battery terminals to see what voltage is there with the engine not running, then do the same with the engine running. You should get readings of approx 12/12.5 Volts and 13/13.5 Volts respectively. Much more than the last figure and the regulator could be suspect
