For info, look at Monkey's post about the jirachi you can get at game stop during the next few weeks. But this post is about what it will do in Heartgold/SoulSilver.
As you know, PCP (pikachu-colored-pichu) unlocks the spikey-eared pichu in HG/SS. Jirachi has confirmed to unlock a special pokewalker course in HG/SS, once traded there from 4th gen games.
http://www.serebii.net/heartgoldsoulsilver/pokewalker-area.shtml#edgeofthenightssky
Those are what is available on the course unlocked by jirachi. Be sure to pick yours up before March 13th !