For the I2C interface check the Philips I2C specification document on their
website. It shows level conversion being done with just 2 FET's. I have not done
it but others have reported on this list that it is really that simple to do

>Another possibility might be to run the PIC at a lower voltage, but there
>will be other components driven by it, including an RS232 serial line driver
>which must have 5v as it generates +10 and -10 from the 5v supply current.

Use the MAX3232E as the RS232 converter. It will operate from 3V and give the
+/-10V out. Then your only problem is to see if the PIC will work at a fast
enough clock rate when run on 3V.

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