Leo,
8051 outputs are open drain so when you write a "1" to a port pin then simillar to a open collector output the output pins simply float as the output transistor is off. the output voltage goes high due to the pullup resistors from Vcc connected to the output pin. When you write a "0" to the port pin then the transistor goes on and thus it conducts and grounds the output pin.
Before reading a port/pin you have to write 1 to that pin in order to make the output transistor float i.e. go off so that you can read the external input.
When you read the port pin you will read 0 since the output device makes the current flow from the 8051 port pin to device as device is pulling low the port pin.
You need to read the 8051 bible links given by erik
Regards,
Prahlad Purohit