Hi,
do you use T89C51RD2 or AT89C51RD2 ?
The reason I ask it, is that newest Atmel revision of this chip (called AT89C51RD2) has built-in Power Monitor. As result it generates internal reset when power-on and power-fail. It holds reset while OCS is not stabilized as well. Theoreticaly you may not use any external reset circuit for power-on condition. But... Who knows, how the power comes; use a power SV anyway and be happy (=
Good days!