"Can I2C or SPI be implemented in RS-485 environment ?" yes, you CAN (I would'nt) have the same chip running 485, I2C and SPI.
"in case I want to place many device in quite long distance using 8051." There is a philips appnote (you know now how to find them) titled something like "mile long I2C" so that is possible, however, the "regular" way to distribute devices is RS485. RS485 has the advantage of being a differential bus and thus (theoritically) immune to noise. For RS-485 info, go to http://www.national.com/apnotes/Others__v7.html and download appnotes 972, 903 and 216.
If you do not have a noise problem, you might try the "mile long I2C", I never have (being burnt by noise too often), otherwise RS-485 will do the trick,
Are we having fun yet?
Erik