I agree that the use of a transistor as a sensor input like this is a great idea. Particularly if the sensor is located remotely from the location of the 8051 with the transistor receiver. A long wire can pick up EMI and ESD to which a transistor is far more likely withstand. The CMOS gate recommended by Erik is not the best solution when a long wire input is a possibility unless additional protective components are designed in to the circuit.
Mike Karas