I don't know if it was just a glitch but...
Put a level 60 gorebyss (male) in the day care centre with a level 100 Groudon. Wait a while and the man will come out and give you a mystery egg, ride around on your bike until it hatches into a jirachi.
I have a level 86 Jirachi at the moment and it is very good, the only downside is that because it is steel and psychic, this means that ground moves are super-effective. So watch out for earthquake because it is a very good move AND super effective!!
Added by: chuck_404 Jul 27th 2008, ID#12265