Looking at the crosstabs here - Democrats support McCaskill with 97%, with Akin not even getting 1%. Only 3% undecided. However, with Republicans, 9% remain undecided. Akin leads 42-36 amongst Independents, with Dine getting 9% (unlikely to actually occur), and 13% undecided. Mitt Romney leads with Independents 57-34.
This leads me to think that most undecided voters, almost all who are either Republicans or Independents who support Romney, likely lean to the right, and are currently battling on whether to vote on who they like personally or who they agree with politically. It appears that undecideds should break towards Akin, so I think this race could be close like 2006.
Of course it'll be relatively close. But Akin has a hard path to an actual plurality.