This problem is addressed in ArcGIS 8.1 Service Pack 1. See Related Information section to download this pack.
Networks in a PGDB with a weight assigned to the Shape_length field on an edge feature class will have corrupt weight associations on paste to SDE.
The network cannot be registered as versioned and the particular feature class cannot be edited.
On paste to the SDE Geodatabase, the weight is not associated with the shape field for the network feature class.
Drop and rebuild the network, associating the weight with the new shape field.