Shapefiles produced by the GeoProcessing Wizard in ArcMap do not have length or area attributes calculated for the new shapes. If the input shapes have these attributes, the output will inherit the existing values of these attributes.
This also applies to Geoprocessing tools and shapefiles in ArcGIS 9.
This is a known issue with the GeoProcessing Wizard. The shapefile data structure does not support the automatic calculation of spatial attributes, such as area and length. The problem is that these attributes could be in any field, and the wizard could end up overwriting something that you do not want changed.