As far as I remeber, AT89c51 is not quite in system (re)programmable. The app note about programming it in system builds a complete AT89c51programmer circuit on the target AT89c51 board and some more circuitry to switch the AT89c51 from the programmer to the target and vice versa. I beleive that any flash part can be programmed this way, and this is not an acceptable form of in sytem programming in my book. I also think that claiming the part itself has this feature is somewhat misleading.

The AT89S51is a more expensive part and offers true in system programming via a simple serial link, and this is what I would call in system programming, in circuit programming or whatever it is...

I hope this helps.

Dincer