From the ArcGIS online help under the "Data Frame" class, DataFrame example 4 shows you how to do this:
The following script converts a data frame's extent object into a
polygon feature so that it can be used with the
SelectLayerByLocation_management function.
import arcpy
mxd = arcpy.mapping.MapDocument(r"C:\Project\Project.mxd")
df = arcpy.mapping.ListDataFrames(mxd, "Transportation")[0]
lyr = arcpy.mapping.ListLayers(mxd, "Accidents", df)[0]
#The DataFrame extent object is converted into a polygon feature so it can be used with the SelectLayerByLocation function.
dfAsFeature = arcpy.Polygon(arcpy.Array([df.extent.lowerLeft, df.extent.lowerRight, df.extent.upperRight, df.extent.upperLeft]),
df.spatialReference)
arcpy.SelectLayerByLocation_management(lyr, "INTERSECT", dfAsFeature, "", "NEW_SELECTION")
mxd.save()
del mxd