I haven't really tried to assign numbers, but I think your assessment is pretty realistic for two reasons. 1) There are a lot of white voters/Catholics in the undecided column. It's a decent bet that a good chunk of them pulled the lever for Bush. KESG has been playing with the numbers and can perhaps go into detail. 2) The Obamamedia has made this election a referendum on Obama. All Obama, all the time. Plus an unprecedented amount of advertising sparked by HUGE fundraising. So in essence he's become the "incumbent" in this open race. Unless the Dems have a really good surprise (a la the 2000 DUI news) up their sleeves, I don't see him getting a majority of undecideds.
Reading the internals of several polls together, most of the remaining undecided voters are white, older voters who went heavily for Bush in 2004. There also seems to be a smaller but perhaps significant chunk (depending on which state) of Hispanic Catholics who also favored Bush in 2004.