This problem is addressed in ArcGIS 8.1 Service Pack 1. See Related Information section to download this pack.
Feature layers created with the Add Route Events and Display Route Events dialog are always visible, even when ArcMap is customized to not display a layer when first added to the display.
New feature layers created by these dialogs don't honor this setting and will always be displayed when first added to ArcMap. This is a known limitation.
There is no alternative solution within the ArcMap GUI. If you are adding layers programmatically, you can have your code choose whether or not they will be visible.
Refer to the Dynamic Segmentation samples in the ArcObjects Developer Help for more info on adding layers programmatically.
The option to set whether new layers will be visible when added to ArcMap is available in ArcMap under Tools > Options. Select the Application Tab and uncheck the box next to 'Make newly added layers visible by default'. New layers will then be added to the Table of Contents but the features will not be displayed.