Unless I'm mistaken, a default only happens if we don't pay our creditors that service our debt. For August that's supposed to be around $29billion [1]. We have more than enough to cover that (the article predicts ~$150 billion in revenue). Every other expense is legally optional and failing to pay those is in no legal sense of the word a default.

[1] http://about.bgov.com/2011/07/12/august-invoices-show-u-s-tr...