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[Reply #15 on:]

Bottom line, it's clear to me at least that any 328p circuit using a series cap coupled auto-reset feature should have the reset diode installed for best reliablity.

Yes, I agree.  I will be using the diodes on all my boards going forward.

I should have been more careful with the wording in my previous post in this thread.  It would have been more accurate to say that on this particular board and with this particular chip, the latch up problem wasn't exhibited using the Duemilanove bootloader.

@retrolefty:  per your suggestion on that original "diode thread" I downloaded Bray's terminal (very nice application).  I saw the same behavior on my boards (optiboot bootloader) that you reported, i.e., that a reset is triggered by any change in the DTR line when no diode is installed.  After installation of the diode a reset only occurred when DTR went from low to high.  No reset is triggered by changing DTR from high to low once the diode is in place.

[Reply #16 on:]

The documented minimum voltage to enter HV programming mode is +11.5vdc on the reset pin

Thats most likely the minimum voltage to reliably  enter HV programming mode.
It may be possible that some chips or certain other conditions may cause it to enter HV programming mode with a lower voltage.
Bottom line is that the voltage on any pin should not exceed 5.5v for reliable behavior.

[Reply #17 on:]

The documented minimum voltage to enter HV programming mode is +11.5vdc on the reset pin

Thats most likely the minimum voltage to reliably  enter HV programming mode.
It may be possible that some chips or certain other conditions may cause it to enter HV programming mode with a lower voltage.
Bottom line is that the voltage on any pin should not exceed 5.5v for reliable behavior.

More properly the +11.5vdc is the datasheet guaranteed minimum voltage to enter the HV mode. I am convinced that +10vdc pulse on the reset pin is responsable for the latch up the chip some of the time in some situations, it is the root cause. I feel that now that it is a known failure mechanism, however seldom or randomly it might be, the fix is just too simple to ignore, either add the diode or don't use the auto-reset cap. Certainly that is why the Arduino folks added the diode in the UNO R3 board.