If there's a default, there will be a lot of finger-pointing, but most of the blame should fall on Republican leadership in the House. TPM's Josh Marshall equated the negotiations between the House, Senate, and administration to a game of chicken, except one of the cars has no driver. No one knows if Republicans have the votes to pass Boehner's, Reid's, or anyone's plan.
Everyone assumes that this is because a large number of congressional Republicans cannot be trusted to raise the debt ceiling. They're even on the record saying things to that effect. But Boehner's inability to control his caucus is his fault.