Hallo José,

why not using an emergency power supply, which delivers your circuit with supply voltage in the absence of mains voltage? Have a look at this thread:

http://www.8052.com/forum/read.phtml?id=71623

About mains voltage measurement:
Keep in mind, that mains voltage is delivered as RMS voltage. If mains voltage supplier guarantees 'his' voltage to be within 230VAC +-10%, then this is a RMS voltage, NOT a peak voltage and NOT an average voltage, either!

As measuring the peak voltage or an average voltage can give totally eroneous results, the measuring of true RMS value is really essential. So, you should use a separate tiny safety transformer (safety class II, providing double or enhanced isolation), with a pure resistive load connected at secondary side. If resistive load is made as voltage divider you can directly connect an AD536, for instance, for fabricating true RMS voltage.
Of course, don't forget to use some protection circuitry: A varistor across primary winding and a bidirectional transzorb across secondary winding. Also, a little low pass filter should be introduced at input of AD536, providing a corner frequency of let's say 5kHz.

It's not wise to use the original transformer supplying your digital circuit for this purpose. Why?
Because these rectifier diodes cause a sudden limiting of secondary voltage, if they turn-on. So, this would give you a rather unpredictable estimation of mains voltage.

How to calibrate this circuit?
Apply mains voltage of known RMS value and measure output voltage of RMS converter AD536. Then, either adjust the voltage divider at secondary winding, or correct the deviation by the help of your 89C51 software.

How to find out the correct RMS value of applied mains voltage?
Very easily, by the help of standard DVM (digital voltage meter): Many of them show the true RMS voltage of AC signal! Have a look at according manual.

Kai