import arcgisscripting, string, os, sys
gp = arcgisscripting.create(9.3)
gp.Workspace = r"U:\GIS\Temp"
ascList = gp.ListRasters("*.asc", "*")
for ascFile in ascList:
    dsc = gp.Describe(ascFile)          # Gets the Description about the ascii file
    ascIn = dsc.CatalogPath             # Gets the Full Catalog Path of the ascii file
    ascOut = ascIn[:-4] + ".img"        # Removes the .asc extension and adds .img
    gp.ASCIIToRaster_conversion(ascIn, ascOut, "FLOAT")
					
				
			
			
				
			
			
				
			
			
			
			
			
			
		ArcGIS doesn't see .asc files as files, they are recognized as raster datasets. arcpy.ListRasters() would have worked in this case.