Simon,
Logging, the type described from the Operations menu is there to log the values of the front panel items to a text file for later retrieval. You can log once when the VI completes (great for viewing subVI test results at a later time) or interactively. It is not meant as a redirection system. That kind of functionality can be programmed into the VI using case structures and a boolean switch.
For more information, look up 'datalogging' in the LabVIEW Help.
Randy Hoskin
Applications Engineer
National Instruments
http://www.ni.com/ask