Scott - since I don't need a particular timeout for this application, why
wouldn't INCFSZ work.  That is:

    clrf    c_low
    clrf    c_mid
    clrf    c_high

s1:
        BTFSC   IOPORT,IOBIT
         goto   went_high

        INCFSZ  c_low,F
         goto   s1

        clrwdt                  ;

        BTFSC   IOPORT,IOBIT
         goto   went_high

        INCFSZ  c_mid,F
         goto   s1

        nop

        BTFSC   IOPORT,IOBIT
         goto   went_high

        INCFSZ  c_high,F
         goto   s1

time_out:

Note also that your routine lets me take care of the watchdog with no added
penalty!

Now, since incrementing each added byte of counter causes the previous bytes
to miss a count (the mod 257 that you alluded to in your message),
correcting to mod 256 should be easy.  The modulus changes as you extend the
counter: in the 3 byte counter above, the lowest byte misses the counts
accumulated by both of the upper bytes; the mid byte misses the counts
accumulated by the high byte.  A first pass of the correction routine
follows - but its pretty stinky and I'm sure that I am missing something
obvious.

went_hi

    movfw       c_mid       ;add mid byte to lower; propogate carries
    addwf       c_low,F
    movlw       1
    skpnc
      addwf     c_mid,F
    skpnc
      addwf     c_high,F

    movfw       c_high      ;add high byte to lowest; propogate carries
    addwf       c_low,F
    movlw       1
    skpnc
      addwf     c_mid,F
    skpnc
      addwf     c_high,F

    movfw       c_high      ;add high byte to mid; propogate carries
    addwf       c_low,F
    movlw       1
    skpnc
      addwf     c_mid,F
    skpnc
      addwf     c_high,F

Back to you, Scott (and the rest of the PicListers).

ideas, improvements?

dwayne