Unsustainable is a strong word for describing state budgets. The three worse states going by credit rating are Illinois, California, and New Jersey. Each of them has a better credit rating then Greece.
First you need to understand why Greece defaulted and not Spain or Portugal. Greece is a mess (technically speaking). First, Wiki tax evasion and corruption in Greece. The infrastructure for a sustainable tax revenue is just unrealistic.
Second; Greece, Spain, and Portugal (and you may even say Iceland, but they are not part of the Euro and had other tools in their belt) were in the same boat after the financial crisis. Spain and Portugal took their medicine. They adopted austerity policies that gave them flexibility. Greece did not. When the Euro experienced a double dip recession, Spain and Portugal's previous reforms gave the markets confidence in them. Greece inspired no confidence.
To compare Greece to the states, Anton's description of the feds fiscal policy is dead on. Instead of trying to generalize all three, I will focus on one, New Jersey. New Jersey has the third worse credit rating for any state. The reason for this is purely political. New Jersey is number 8 in GDP by state (Illinois is 5 and Cali is 1) and 11 in population (Illinois in 5 and Cali is 1). The media in NJ consistently talks about a bloated pension system. While the system may be high, it is not unsuitable. The real problem is the political dead lock between the governor and legislator. With out picking sides, both have failed to finance the budget. Instead they choose to sell assets to fill budget deficits. An example is a lawsuit they won against cigarette companies that pays NJ money every year. NJ sold this to fill a gap in the budget. Actions like these makes the markets lose confidence and pushes NJ closer to default.
That being said, NJ problems and Greece's problems are two different beasts. For all NJ short comings, they have a massive tax base that actually pays taxes, a responsible and effective legal system, the US FED, non-corrupt politicians (but not effective), New York City, and industry. Greece has none of these things ( I am still not sure what Greece exports other than unfinished goods).
The second question was what could be learned.
You should read this paper written in 1999.
is there a point where the Feds will step in and take over all the responsibility of the state, ensure that a state cannot go bankrupt ?
The Federal Bankruptcy law does not allow states to go bankrupt. I am unaware of a time where the Feds had to step in to float a insolvent state. California issued iou's to people in 2009 but received no help. Remember that for most of US history, good practice was considered to have a balanced budget. It was not till the mid 20th century that states started to take on serious debt.
If a state was to be seriously insolvent, the Feds would have to bail them out. Now, the Fed doesn't have to legally, but to save the economy from a dip it would (just how it bails out financial institutions). It would most likely be a buy bonding program and the federal courts and federal administrations would be heavily involved with things such as pension reform or whatever the case may be.
As to what Europe can do from the fed model:
Read the paper. A brief explanation is that in America, the citizens consider themselves Americans, not New Jersians or Californians. They are willing to accept the rule of a federal system. In Europe this is not the case. A citizen of Germany considers herself a German, and a person from Italy an Italian. Neither of them would be willing to give up their independence to submit to a federal system the way the states do. For the EU to work, they would need complete separation from the National governments. For example, Merkel would not be involved in the Greek bailout talks (she would be more of a governor), France's legislator would not vote on it (it would be a state legislator), and Greece would have to accept whatever the EU says (because all Federal law trumps State). The people of those countries would have to appoint a leader (the German word is Führer) and Germany would have to accept an almost equal say in things as Malta (just as NY has just as many senators as Alaska). I do not know how realistic that is, and history suggest that it would not be peaceful.
And as a side note, Greece is like Argentina. Every person who bought a bond from Greece knew it would default, it was just a matter of time. This is not the first or last time Greece will fail.