In response to questions from people everywhere, I will share a couple of quick thoughts on the possible departure of Greece from the euro. First, several people have raised the possibility of Greece being thrown out of the euro.
There is no way that Greece can literally be thrown out of the euro in the sense of being prohibited from using the euro. Any country has the option to use any currency it chooses. This was an issue that came up in the referendum over Scottish independence. The independence movement wanted to leave the United Kingdom but to continue to use the British pound as its currency. U.K. Prime Minister David Cameron said that the Scots could not keep the pound if they left the United Kingdom.
This was not true, unless the U.K. was prepared to invade Scotland and physically prevent their banks and stores from using the pound. The Bank of England could refuse to support any of the Scottish banks, which would make it highly undesirable for them to use the pound, in addition to the fact that the U.K. would not be setting monetary policy for the benefit of Scotland, but Scotland would certainly have the option to continue to use the pound for their currency.
In this vein, there are several countries around the world that use the dollar for their currency, including Panama, Ecuador, and Zimbabwe. They did not need to get permission from the United States to use the dollar, they just opted to do it (in the case of Ecuador and Zimbabwe to end hyperinflation).
In this way, Greece will have the option to keep the euro indefinitely. It is difficult to see why it would want to if it lacks the support of the European Central Bank, since it would almost certainly mean a substantially worsening of its economy from its current Great Depression levels of output. However if Greece's leaders decide that keeping the euro is more important than reviving the economy, the eurozone authorities cannot keep them from doing it, short of an armed invasion.