To make use of incremental rebuild, it needs to be an existing geodatabase that has been upgraded to be a 10.0 geodatabase and with the network dataset upgraded to be a 10.0 network dataset (using the "Upgrade Network" geoprocessing tool). Or, new network datasets created on 10.0 geodatabases should have this new capability by default.
In all cases, the first time you build the network dataset, it will do a full build of the entire network. And schema operations that completely invalidate the network dataset will still require a full rebuild. However, if you subsequently just edit a few features, it should track dirty areas and when you then click to build the network it should be significantly faster since it just rebuilds the network for the area covered by the dirty areas.
