if you see a crack forming that doesn't have gobs of structural 
tension on it, JB Weld can work, as Tim Smith points out.  it may 
stop the oil leak and get you many more miles.

to do an effective repair, locate the beginning and end of the crack
if at all possible.  then, and this is the sickening part, drill a
small hole at the ends of the crack.  this relieves the stress line
formed by the separation the metal in the crack and helps
stop the crack from traveling.  the trick is to get the hole
juuusssst at the end of the crack so that the crack would terminate
about 1/2 the way into the hole.  then grub out along the crack with
a hand grinder to get a spankin' new metal surface.  fill 'er in
with JB Weld and pray.

-dan (who's cracked a few motorcycle casings during his 'formative' 
years)