the information about this election keeps moving around--a few days ago ahmadijenad was said to be heading toward a crushing defeat, so much so that the ultra-right clerics were trying to get him to stand down from the elections--then there was a television debate--then pissiness following on that---meanwhile, there are two different types of campaigns between the two main candidates, not alot unlike the mc-cain/obama thing.
personally, i don't know enough about mousavi (sadly at the moment, i don't read farsi)...and the english coverage seems spotty: so here's a couple links to stuff in french.
from le monde on the election (a slightly expanded perspective on tomorrow's doings):
Les réformistes iraniens espèrent chasser Ahmadinejad - Le Monde.fr
from le monde diplomatique, a web-article about the election itself (better context if you chase the linked articles):
Présidentielle iranienne : Ahmadinejad contre les « pragmatiques »