President Obama has a credibility problem. He has compromised so often that Republicans simply don't believe that he'll sustain his opposition to anything, as this exchange attests:

Rep. Henry Waxman (D-Calif.) bluntly asked Obama whether he was willing to fight for Democratic priorities amid GOP calls for trillions of dollars in spending cuts.
In asking the question, Waxman said he’d asked several Republicans about their White House meeting the day before and had been concerned by their response. 
“To a person, they said the president’s going to cave,” Waxman told Obama, according to his colleague’s account.
“If you’re not going to cave, eliminating that misunderstanding is very, very important to the negotiations,” the lawmaker said, retelling Waxman’s message. “And if you’re going to cave, tell us right now.”

That problem can be seen in the fact that Republicans have convinced themselves that John Boehner's debt ceiling plan can pass the House, pass the Senate (how it would gain 60 votes I couldn't say) and then force Obama to sign it. Former Republican staffer Keith Hennessey writes:

The president’s speech was notable only for what he did not say: “I will veto the Boehner bill.”
By not threatening a veto of the Boehner bill, the president leaves the door open to signing it.
The question now is whether 218 House Republicans can vote for and pass the Boehner bill before the president regains his footing.

It's true that Obama didn't use the word "veto." But he did say this:

But the new approach that Speaker Boehner unveiled today, which would temporarily extend the debt ceiling in exchange for spending cuts, would force us to once again face the threat of default just six months from now.  In other words, it doesn’t solve the problem.
First of all, a six-month extension of the debt ceiling might not be enough to avoid a credit downgrade and the higher interest rates that all Americans would have to pay as a result.  We know what we have to do to reduce our deficits; there’s no point in putting the economy at risk by kicking the can further down the road.
But there’s an even greater danger to this approach.  Based on what we’ve seen these past few weeks, we know what to expect six months from now.  The House will once again refuse to prevent default unless the rest of us accept their cuts-only approach.  Again, they will refuse to ask the wealthiest Americans to give up their tax cuts or deductions.  Again, they will demand harsh cuts to programs like Medicare.  And once again, the economy will be held captive unless they get their way.
That is no way to run the greatest country on Earth.  It is a dangerous game we’ve never played before, and we can’t afford to play it now.  Not when the jobs and livelihoods of so many families are at stake.  We can’t allow the American people to become collateral damage to Washington’s political warfare.

That doesn't sound like he something he can sign, does it? I've been pretty shocked by the compromises Obama has been willing to make, but signing the Boehner bill would be a whole different level of capitulation. (Read Bob Greenstein's summary to get a sense of how radical the Boehner bill is.) Alarmingly, Gene Sperling declines to use the word "veto" when asked how Obama would react to the Boehner bill.

Obama clearly faces a perception problem. Republicans may complain that he's walked away from deals, but they really think he's a pushover who will cede more and more ground the harder and longer they push. That's a dangerous position to be in on the verge of a high stakes game of chicken. It encourages the Republicans to push the envelope farther and farther -- even to walk away from a deal they regard as a win in search of an even better win.

And, of course, if Obama did somehow agree to the Boehner bill, the problem would get vastly worse. Republicans would simply treat the next debt ceiling hostage fight as a chance to increase their demands more. Indeed, if Obama capitulated to Boehner this time, can you imagine the negotiating dynamics that would occur when the next debt ceiling vote comes in 2012? It's almost impossible to imagine the upper bound of the Republican demands by then.

Right now the Boehner bill is gaining momentum. As awful as it is, if Senate Democrats sense that Obama is going to undercut them and capitulate, they may be tempted to get ahead of him and endorse it. He needs to threaten a veto immediately.