I haven't seen this anywhere else, but according to Ryan Grim of the Huffington Post, President Obama referred Dennis Kucinich to a hidden provision in the Senate Bill that allows for a single-payer option several years down the road:
"But, said one attendee, Obama pointed Kucinich toward single-payer language that Sen. Bernie Sanders (I-Vt.) was able to get into the bill. Kucinich fought for an amendment that would allow states to adopt single-payer systems without getting sued by insurance companies. Obama told Kucinich that Sanders's measure was similar but doesn't kick in for several years. "He definitely wrote it down," said one member of Kucinich, suggesting that he'd look into it."
Maybe, just maybe, this is the real reason Kucinich changed his vote.