The 'official' way is to use buffers (74lvcx I think..) to translate between 5/3v but there are other techniques.

For 8051's that have a weak pullup to 5v you can use a resistor (say 10k) to pullup the port pin to 3.6v and off you go - cmos 0/1 logic level is normally 1/2 vcc (1/2 5v = 2.5v) so to read the port pin you are fairly safe - just don't expect it to work over a length of flat cable or suchlike. For devices on 5v that have an active drive to 5v you can use a resistor voltage divider and a small amount of capacitance to speed up the edges.