Software Update service supports only pkm.en file types, recognized only by
Mac OS X v10.4 and later.
Page 76 System Imaging and Software Update Admin Manual 10.5 available from here:
http://images.apple.com/server/macosx/docs/SystemImaging_and_SW_Update_Adminv10.5.pdf
So yes to 10.4 and 10.5 clients but not 10.3.
Tony