In terms of California bond holders actually missing a payment, those odds are remote. S&P points out in their report coinciding with putting CA on negative watch:
An austere analysis of the state's ultimate capacity, from a budgetary perspective, to service its debt suggests to us that at $35.97 billion, constitutionally required spending on education (Proposition 98 expenditures) for 2010 leaves $53.15 billion in resources available for debt service (estimated at $5.74 billion) on general obligation and lease revenue bonds.
So if it came down the state actually running out of cash, first certain education spending would be met, then bond holders. On that basis, there is plenty of coverage. I think this leaves the likelihood of a payment completely missed as extremely low.
Now a payment delayed is a different matter. If the legislature were to not pass a budget by June 30, the state wouldn't be able to sell short-term notes to restock their checking account. At that point, I really don't know what the protocol would be. If, in theory, the state literally ran out of money, they obviously couldn't forward coupon payments on to bond holders. This would be a form of default. But of course, bond holders would eventually get their payments, most likely when the next quarterly payments were made by tax payers.