hi,
First of all, we talk about defined chip called AT89C51, yes?
Well, ALE may be used to produce clock with 1/6 of OSC frequency. Note: it is correct as long as you do not use MOVX instructions - in this case, second ALE pulse is ejected.
As about PSEN. If a program locates in internal flash only then PSEN may be activated with MOVC instruction while the address is above internal flash limit (here: 4kb). If you have not used external program memory then PSEN may be used for debug purposes: it must not be activated while normal execution inside internal flash memory. Unfortunatelly, when you set lock bits and so do protect code fetch from external memory then PSEN will not be activated even if program jumps over flash memory limit (BTW, I have not tested such condition so try yourself - who knows maybe it activates anyway due another Atmel yet not-published Errata :)

Regards,
Oleg