I think the schematic is just wrong.
I've attached two LTSpice simulations.
a) original schematic from MEGA 1280 board with 2 capacitors. This is just wrong. With the additional capacitor to ground, you can only get down to Vcc/2 with two identical capacitors. To get down further, a bigger C1 is needed. This is just like charging two identical capacitors in series. The middle point is Vcc/2.
b) how it was done on the Duemilanove board. NO capacitor to ground, works perfectly - in simulation and reality.
reset_bad.asc (1.01 KB)
reset_good.asc (886 Bytes)