http://www.atmel.com/dyn/resou...OC0313.PDF states under "absolute maximum ratings 16 mA. In the DC charactersistics it states that the output low will not exceed 0.45V at 1.6 mA. These are the kind of values you will find for most chips.
So what is this good for? Well, it gives you at the Vol at a high current for driving logic: 1.6 mA (which is actually what the chip is intended for). It also states tht if you design something where the port must drive more than 16mA Atmes is not responsible for the chip blowing up in blue smoke.
Anyhow, with the exception of driving a small LED only, the port pins should never be used for driving anything but logic chips.

Erik