Stephane,
In my earlier P89C51RD2 boards I had used a MCP810 supervisor with a transistor to invert it.
In my present board I am using a TL7705 (with lots of caps).
I had no trouble with MCP810 and transistor combination, and right now I am repeatedly programming and power cycling the TL7705 and the P89C51RD2 with equally good results so far I have not lost a single micro.
I am usually a fan of the DS1232 (ADM1232, MAX1232) unfortunately since the built in watchdog timer cannot be disabled during development process I could not use it in this design.

Regards,
John.