interestingly the resistor can be left out with no problem
Depends on the electrical environment. Atmel recommends one, see Section 3
but the diode does solve issues caused by positive spikes pushing the chip into isp mode on some platforms.
See Section 3 also - High Voltage programming mode is what seems to kick off, leaving the chip in what appears to be a hung state.
i found that connecting the 10k to 3.3v instead of 5v also works for all systems w/o need for a diode.
I'd say that's kind of iffy - Section 29 shows Min Voltage for Reset to be a valid High is 0.9 x Vcc, so 4.5V for a 5V powered part.
VIH2