Once again, this reinforces Iowa's non-wildcard history, which we examined back in June. Iowans may flirt with an outrageous candidate for awhile--in this case bringing her to the top of the polls in the summer and giving her the win in the Ames straw poll--but when voting day approaches, Iowans shift toward electable candidates.
Bachmann's inherent weaknesses as a candidate, particularly her lack of electability, doomed her campaign from the beginning. Her early feud with Sarah Palin didn't help matters. Elephant Watcher never gave Bachmann more than a 3% chance of winning the nomination, and set her to 0% about six weeks ago.
Bachmann's departure should give a slight boost to other Anti-Romney candidates by distributing her votes among them. However, Bachmann was already polling badly in the rest of the early states, so there are few votes to distribute.