a)  Cyrille,  Migeru,  afew, (maybe melo):
We need a credible threat to leave the Euro, to change the European institutional setup. The exact nature of the changes you want has not been spelled out, but from our discussion here it is quite clear that what would be needed is, i) changes to the 'Stability and Growth Pact', ii) The possibility for the ECB to directly finance gouvernments. iii) maybe a higher inflation target.

b)  A swedish kind of death,  cagatacos:
We should leave the Euro right now.

I want to lay out here why I think that position a) is wrong and b) is correct. With the caveat that b) makes most sense for Portugal, Spain and Greece.

Why is a) wrong?

The threat is not credible:
As expressed by afew, melo and Cyrille, I guess the idea here is that if countries where to leave the Euro, their new currency would devalue leading to less demand for German good and in turn to a negative economic impact for Germany.I think there are several reasons why this will not be take seriously in Germany:
I)German economists are crazy and the economists advising our government are the worst of the bunch.
The understanding of the economy in Germany is deeply ideological: 'if you work hard things will be fine', 'a lower value for the currency is only needed for lazy southerners' and so on.
II) Even looking at it objectively it is not clear to me if German would be worse of if other countries would leave the Euro. All other countries have to import at lot of good, no matter what. So the amount of devaluation possible is limited. At the same time, the real income of Germans would rise. they could go cheaper on Holidays, they could by cheaper goods from the rest of Europe and so on. this would create a rebalancing which is urgently necessary.
III) In the special case of France I am not even sure that the new currency would devalue. The French economic weakness is a myth. I am not saying that everything is fine in France, but still the economy is quite strong. Furthermore, in contrast to Germany France has not destroyed its infrastructure in an obsession with its deficit and it has a growing population. These two factors lead to the conclusion that its growth potential is much better then the growth potential of e.g. Germany. If now on top of this France would have its own currency and could run a sane monetary policy this would probably create solid growth (I will also come back to this in b)) and therefore an appreciation of  the currency.
IV) If you simply threaten to leave the Euro your financial system will be destabilized. As soon as you start to talk about leaving the Euro you basically have to shut down all banks, or you will have a bank run. Even if you agree with I.III), then it would still make sense to first take your Euros out of France, wait till the new currency drops a little and then go back in. For Spain and Portugal, anybody how lets his Euros be exchanged to the new currency will have substantial losses. So either you do it directly and quickly, or your banks will collapse and you will have to beg the IMF for Money.

Lets go to b):

We should not underestimate the challenges in leaving the Euro. I also have to say that legally any country leaving the Euro would enter unknown territory. If some investment fond would loose money because your country leaves the Euro, could it sue? Most probably. And, while in the long run leaving the Euro could be hugely beneficial, in the short run it could lead to real loss of purchasing power for imported goods, which are a lot of goods. A debt default is also pretty much unavoidable, which also comes with real costs down the line.

That said: 25% unemployment is unacceptable. I do not see how this can continue! This is an obvious reason why Spain, Portugal and Greece should leave the Euro. Similar to Argentina around 2000, the benefits of a different monetary policy would most probably outweigh all costs.

Now for France the Situation might be different. It could probably leave the Euro, and some of its creditors might even agree to denominate the debt in the new currency. Or maybe not, but even then France will not need to default on its debt. I am quite confident that growth would be sufficient. Again, it is not clear to me that leaving the Euro is really a good idea for France at the moment, but it still beats the 'whining about leaving the Euro' option.