Well, that's strange. It's not what the graphs tell.
Update:
The graph shows 5V @ 0ma and 4.475V @ 20ma.
Since output resistance is given by Ro = Vo / Io we get:
Ro = 0.525 / 0.02 = 26.25 ohms
I also notice that if we just look at the beginning of the curve we have
Ro = 0.125 / 0.005 = 25 ohms
And if we look at the last part of the curve we have
Ro = 0.145 / 0.005 = 29 ohms
So, according to the specsheet, the Ro actually rises as Io increases, which is exactly the behavior we would expect from a pull-up consisting of a small resistor and a teensy MOSFET, which is exactly how the AVR pin is configured for output.
Grumpy_Mike, are you suggesting the specsheet is wrong?