Adding file name to attribute field using Calculate Field?

This python code adds FILENAME field to all Featureclasses (excluding those in Datasets) and populates with featureclass name.

# Import standard library modules
import arcpy, os, sys
from arcpy import env

# Allow for file overwrite
arcpy.env.overwriteOutput = True

# Set the workspace directory 
env.workspace = r"P:\geodatabase.gdb\filename" 

# Get the list of the featureclasses to process
fc_tables = arcpy.ListFeatureClasses()

# Loop through each file and perform the processing
for fc in fc_tables:
    print str("processing " + fc)

    # Define field name and expression
    field = "FILENAME"
    expression = str(fc) #populates field   

    # Create a new field with a new name
    arcpy.AddField_management(fc,field,"TEXT")

    # Calculate field here
    arcpy.CalculateField_management(fc, field, '"'+expression+'"', "PYTHON")

There is a script on the arcscripts website that should assist you in doing this

http://arcscripts.esri.com/details.asp?dbid=14534

Summary A simple script for creating a field called "FILENAME" and attaching the filename of the shapefile to each entry in the attribute table. Use with as many shapefiles within a specified directory. Useful when picking apart shapefile entries and combining into a separate file to maintain a breadcrumb trail of the original shapefile name.

Run file within a directory containing all the shapfiles needing modification

I hope that this's what you're after

Here is a link to an even better and easier tool using model builder.
http://catalog.data.gov/dataset/arcgis-tool-inserts-file-name-into-attribute-table#