MANY people over time on this list have supported the use of the PIC's
    internal diodes to clamp signals to an acceptable level.
    DON'T DO IT.
    The PIC is guaranteed "safe" at up to 20ma protection diode current.
    The PIC is NOT guaranteed to work properly with ANY protection diode
    current.

There's a MICROCHIP App Note (AN521) that uses the internal protection
diodes to allow a PIC to be connected to AC mains voltage using only a (5M)
resistor.  They mention a +/-500uA allowable current (into/outof an input
pin) which is backed up by a number in that "absolute maximum" section of
the specs (for 16C54, in my 1992 databook.)

Has microchip retracted that app note?

(and where are people getting this "20mA" number, which I only see as
maximum output current for an OUTPUT pin, with nothing about the
protection diodes.)

BillW