The significant wildcards the polls don't consider
I suspect that what these polls of Iowa Democrats don't take into account are a few significant wildcards that could sway a very close election.
WILDCARD 1: Youth Vote - Much has been said about Obama's appeal to young people as well as their unreliability as voters. I tend to believe that the Obama campaign may have tried to learn a few lessons from Dean's failure to bring out that vote in 2004 and have tweaked their strategies so as not to repeat the same level of disappointment. Even a marginal or mediocre improvement compared with 2004 favors Obama.
WILDCARD 2: Independents/Republicans - Much has also been said about Obama's appeal to independents and Republicans. Assuming even a small percentage of them bother to register as a Democrat so they can caucus for Obama, that helps Obama. If they are not required to make the extra effort to register with the party, then all the better for Obama.
WILDCARD 3: 2nd Choicers (Sub-15%ers) - I've seen polls showing Hillary way behind with this group. My hunch is that John Edwards & Obama will be fighting for this group and I haven't seen any consistent polling or anecdotal evidence of which one they are breaking for. Obama has atleast an equal chance of garnering this vote.
WILDCARD 4: - This election has been about CHANGE. When Dean ran in 2004, and subsequently Kerry for the general election, there was no real emphasis on CHANGE as an overarching theme. Those were clearly themes we saw during the primaries and/or general elections of '76 (Carter), '80 (Reagan), '92 (Clinton), '00 (Bush). If that CHANGE theme is being promoted by a charismatic figure that CHANGE message becomes even more compelling. If the polls weren't in favor of the change agent all along than often times the polls break in his/her favor in the last few days...usually with the undecideds.
WILDCARD 5: - PC EFFECT - I'm actually surprised that no one has brought this up yet. This is certainly nothing that the Obama campaign has any controll over. What has often worked against black candidates in the private voting booth may actually work in Obama's favor since Iowans have to publicly pledge for their candidate, I suspect there will be a percentage of white Iowans that will feel compelled (just because of the PC culture that has been created over the decades) to publicly pledge for a black man since there is no private polling booth to register what they may really feel.