I must say I found that reference by looking for 'fft AND 8051' with a popular search engine. Many other references can be found using this and other similar keys in different engines.
I suppose that the Intel´s application note which was included in a old handbook cannot be found in the web, but I have not yet tried it.
I need to write an fft code for the 8051 too, probably in the next weeks. I have planned to translate a typical high level C code written for a PC (floating point) into a much more simple (long integer) but faster assembly code for the 8051. I think that I shall use rational (fractional) notation for numbers, lookup tables for sines and cosines, and similar tricks.

Regards,
Alfredo del Rio.