What else, other than insert/update/upsert/delete, can cause "System.CalloutException: You have uncommitted work pending. Please commit or rollback before calling out"? I've search the debug logs multiple times, and there absolutely is not a DML statement executed between the startTest and the failing callout.
Are there any other commands, or perhaps another callout (even though it's only an inquiry), that can cause this? Does anyone know how to get around this problem?