I imagine (allthough I'm not exactly well-versed in this sort of thing) that there'd be a long interim period in which national, new currencies exist on paper and pegged to the Euro. This will give time for a lot of government bonds to reach maturity; for those that do not mature in the interim, they'll be "partitioned" into multiple bonds notated in all the different currencies of the former Eurozone. Wether anyone would want to buy new bonds in Drachme or Lira is of course the problem of Greece and Italy, and noone elses. Or at least directly.
For private debts; I imagine that many will renegotiate and pick a relatively stable currency as a bench mark: say, at this point the debt is worth 100.000 Japenese Yen, and when payment is due we expect payment in [French Francs, Liras, Pesetas etc.] worth the same amount in Yen.
As I said before though, I'm not in favour of disbanding the Euro currency. I imagine the resulting situation will be a lot worse then during or before the Euro.
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