Yea, the Vf spec is of just passing interest, we know that a 5 volt signal will fully turn on the positive clamping protection diode on a input digital pin on a 3.3 volt powered board, what we want to know is how much continous current the diode can withstand, that is the datasheet spec of interest.
It seems as if Atmel is a bit candid about this limitation. There is an AVR application note on zero crossing however, where they use a digital pin directly connected to mains. In this note, maximum continues forward current is quoted as 1mA. The relevant text reads as follows:
The series input resistor is a 1 MΩ resistor. It is not recommended that the clamping
diodes are conducting more than maximum 1 mA and 1 MΩ will then allow a maximum
voltage of approximately 1,000V.
Any voltage higher than 1,000V would probably be spikes or surges. The clamping
diodes are able to handle spikes for a short period of time but not surges. The application
note will not go into how to protect against surges, but simply recommend
implementing protection against surges in the design.