Single SOQL query to retrieve both a Currency field and the converted currency value?

Use a field alias for the aggregate expression. For example:

[select Id, Price__c, convertCurrency(Price__c) ConvertedPrice from MyObject__c]

As far as I can tell, the only way to get at this data is to have at least 2 SOQL queries: 1 to get the data, and 1 to query either the CurrencyType table (for simple multi-currency orgs) or DatedConversionRate table (for advanced currency management orgs), which both provide conversion rates. Once you get the conversion rates, you have to manually perform the conversions in order to achieve the desired effect.