Ben Nelson is unlikely to block allowing debate to proceed on health care reform. His justification is that he fears what kind of bill would result if he did vote no:

If opponents prevent consideration of the revised Senate Finance Committee legislation, Nelson said, Democratic supporters “most likely” would turn to the budget reconciliation process to consider health care.

That maneuver would reduce the number of votes required from 60 to a simple majority of 51, he said. In effect, that would remove any need to respond to the concerns of centrist Democrats or independent Sen. Joe Lieberman of Connecticut.

…
“If the three of us voted against the cloture motion,” Nelson said, “we don’t stop health care reform.”

Instead, the Obama administration would decide whether to try to negotiate an amended bill, he said, or the Senate leadership could “go right to reconciliation.”

If Harry Reid is unable to get the 60 votes for a motion to proceed, it is indeed possible that he would turn to some parliamentary tactic like reconciliation to pass health care reform with a simple majority. Whether leadership directly told Ben Nelson that they would use reconciliation, or Nelson is just using the specter of reconciliation to justify his vote, we will probably never know. Either way, this does stress the importance of having a simple majority option like reconciliation on the table.

If the Democrats were completely unwilling to pass health care reform using some method that only required a simple majority, members like Ben Nelson, Joe Lieberman, and Blanche Lincoln would have complete power. They could force the caucus to change the bill to exactly fit their desires, or threaten to bring it down. With Reconciliation, there is at least the possibility that if they reject the compromise offered by progressive Senators, they will end up with an even more progressive bill passed using reconciliation–since Harry Reid does not really need their votes, and so does not really need to let them have any input at all.

Ben Nelson is still threatening to vote against the final cloture:

If the bill is not amended to his satisfaction, he said, “you can be sure I will not vote for the motion to end debate. I’m not going to do that.”

Ideally, when the time comes, the possiblity that his “no” vote will simply result in an even more liberal bill getting passed using reconciliation will give Ben Nelson (and other conservative Democrats) the justification needed to vote to end debate.