How do I display negative numbers with parentheses? e.g. (1,000)

Formatting can be set for merge fields whose data type is Currency or Number. Available in Ver.2.0 or later.

Merge Field Format

{!Object API Name.Field API Name, Format;Minus Format, Rounding Mode}

e.g. {!Case.TestCurrency__c, #,##0;(#,##0)}

Value: -1234

Result: (1,234)

*In Excel, merge fields cannot be formatted. Set the format from "Format Cells".

Instruction manual