Lasse,
The input voltage can exceed the operating range but must always stays within the Absolute Maximum Range. Beyond the Maximum Range the ESD diodes will become active and could damage the device. Do not rely on ESD diodes to shunt current as this can cause damage to the device. Figure 68 in the datasheet shows an approximate input circuit that show the ESD diodes on the Input circuit.
Specifically, regarding the question of what would be the max input current, I suggest using the Operating range of the device.
Using the following calculation gives an approximate value for a maximum current of ~ 740 uA.
If clock is 100 MHz, Cin = 11 pF (Vin+ to GND), input range is 2.7Vpp differential or +/- 0.675 V single ended
I = c dV/dt ; so max current allowed at the input is 11 pF * (0.675 V * 100 MHz) ~ 740 uA single ended.
Regards,
Anthony