Is there a way to convert the c51 compiler so I can write programs on my mobile phone (a galaxy s2) and then download them onto the atmel at89s51 mcu?
"But I would think that even on a fast tablet or iPhone it's going to run Windows pretty damn slow!"
Of course. If the job really has to be done with a phone/tablet, it would be way better to network to a PC (X-Windows or Remote Desktop) and have the PC do the actual work.