This focus on the U.S. seems to me basically irrelevant to gauging a successful election or not. The real issue is how the Iraqis, who don't give a damn about how the whole thing plays out in Kalamazoo, will interpret their election--I repeat, their election. How many Iraqis vote is far less important than the fact that a truly Iraqi parliament will emerge from the process to write a constitution (which will indeed spur the "insurgents" to escalate their bloodletting, since nothing worries them more than the threat of a potentially legitimate Iraqi--not U.S.-appointed--authority).
Yes, Sunni participation is an important issue, but not in the way people presume. If a post-election regime can shape a compromise system that gives all Iraqi communities a stake in the new political order, then two things may well happen: the aftereffects of a low participation level may soon be erased by the more urgent matter of communal compromise; and the fearful Sunnis may begin disagreeing with themselves over how to deal with the new authority. Already there are several reports of Sunni election boycotters who have made it clear they intend to negotiate with a new post-election parliament and regime. America, for them, is completely secondary at this stage.