Answer
Yes. When using ArcView 3.x or ArcMap 9.x, you can edit shapefiles that are currently used by an ArcIMS 3.1, 4.x, or 9.x Service.
However, the following holds true for earlier releases:
When using ArcMap 8.x, shapefiles cannot be edited when they are used by an ArcIMS 3.1, 4.x, or 9.x Service.
When using ArcIMS 3.0, shapefiles are locked and cannot be edited by ArcView 3.x, ArcMap 8.x, or ArcMap 9.x.