In further investigation, I have traced the problem to the following:

I retrieve the information on the opportunity and its current pricebook through this query:

o = [
    Select o.Id, o.Name, o.Amount, CurrencyIsoCode,
        o.Pricebook2Id, o.PriceBook2.Name,
        (Select Id, OpportunityId, SortOrder,
         PricebookEntryId, PriceBookEntry.Product2Id,
         PriceBookEntry.Product2.Name,
         CurrencyIsoCode,
         Quantity, TotalPrice,
         UnitPrice, ListPrice, ServiceDate,
         HasRevenueSchedule, HasQuantitySchedule,
         Description, HasSchedule
         From OpportunityLineItems)
    From Opportunity o
    where id = : apexPages.currentPage().getParameters().get('id')];

This works fine.

The problem is that I was displaying the pricebook name on a VisualForce page with an <apex:outputField>.

Apparently there is a change in Summer '13.

In Spring '13, this works:

<apex:outputText label="Current Price Book" value="{!o.Pricebook2.Name}"/>

In Summer '13, the page fails to load with the error:

Validation Errors While Saving Record(s)
There were custom validation error(s) encountered while saving the affected record(s). The first validation error encountered was "Currency ISO Code: invalid currency code: 000".

If I change the tag to an output text, i.e.:

<apex:outputText label="Current Price Book" value="{!o.Pricebook2.Name}"/>

Now there is no problem, and it works fine.

I just want to know why.