I have a feeling that a lot of the political handicapping of the possibility of a shutdown is getting the dynamic a bit wrong. You see a lot of speculation about the possibility that many House Republicans will defect from a deal to cut spending by $33 billion, and keep it from getting through the House. That's probably not the real impediment. If the White House cuts a deal with Republicans, it will almost surely be able to muster enough House Democratic votes to offset the Republican defectors.
The issue is whether arch-conservatives in the House decide they oppose the deal strongly enough to depose John Boehner as Speaker. If that happens, or appears likely to happen, then Boehner will shut down the government. If, on the other hand, conservatives merely decide they need to posture against a deal for the sake of appearances, then they'll vote no, and Boehner will pass his bargain with help from Democrats, like he did te tax cut deal las December.
The strongest sign of a possible shutdown is that Boehner appears afraid to be seen as dealing with President Obama:
To illustrate just how tense the situation has become, Boehner (R-Ohio) has not committed to a meeting President Barack Obama plans to host at the White House on Tuesday — the same day House Republicans plan to release their entitlement-slashing budget plan for the fiscal year that begins Oct. 1.
There could well be some other factor at work here -- such as Boehner not wanting to be filmed at the White House the same day the White House is attacking Paul Ryan's 2012 budget plan. But if the dynamic is that Boehner can't risk being seen as cooperating with Obama --if he thinks any deal will be seen as a shady "backroom deal" -- then he's going to have to shut down the government. You can't keep the government open without making a deal with Obama.