The Sunni who were essentially invited to help draft the constitution in the hope that they might legitimize it are now apparently "uninvited" when it comes to hashing out the final details. They apparently don't want to sign off on it, believing that they will get a better deal if they wait and refuse the constitution at the next elections. They're now encouraging other Sunnis to vote, apparently in order to more firmly reject the position of the Kurds and Shi'ites.
What this will mean is more waiting, more violence, and an increased risk of forces within the Kurds and Shi'ites to break off and try to go the seperatist route. Elections will get very dicey, as the Sunni need to be the majority vote in several regions in order to reject the proposed constitution. This, to me, says "large turnout" combined with "violent repression of non-Sunni voters". Likewise, the government and the Shi'ite paramilitary groups might get in on the act, trying to violently surpress the Sunni.
On the plus side, it could lead to an actual, legitimate Iraqi election which could strip power from a lot of relatively illegitimate Iraqi leaders... but I suspect the final outcome will still be highly polarized. Civil war lite.
If you factor in this story from NPR about the risk of civil war in Iraq, and the stories about what is essentially ethnic cleansing going on in Baghdad, then it becomes much more obvious what could happen if people start viewing the upcoming election as a deadly serious issue that must be won at all costs.
The decisions being influenced by the referrendum on the next constitution could be very big indeed, as they may decide several major issues, such as whether there will be a Kurdish state one day or not... or whether there will be a wealthy Shi'ite state... or how much political control Iran will exert over Iraq... or whether Iraq will be a secular or fundamentalist state... or whether women will still have a significant role in Iraq's future... or whether oil revenues will be shared evenly... or whether someday, it won't.
Big issues, and so far, not a lot of meaningful compromise -- and, without it, there is a real risk that the next election could further inflame the violence.