As well as what every one else says apart from the resistor power ratings stuff:-
but since people say datasheets tend to be wrong, I'm aiming at 900mA.
Data sheets are not wrong but you need to understand them. That 1A value is given under absolute maximum but here is a note:-
Note 1: Pulse width ≦100μs, repetitive frequency = 100 Hz
This means to achieve this the LED needs to be on less that 0.1mS and off for at least 10mS, that is a 100 : 1 duty cycle. Can you ensure this happens? This duty cycle means you can also reduce the power ratings of the resistor to 1/100th
of the constant current values.
Since the arduino pin supplies 20mA, that means I'll need a resistance of around 172ohms. Correct?
No it will only supply 20mA if you let it. You only need to use a 1K resistor or so, it is not critical.
To get 900mA you need at 5V to have a series resistor of:-
Supply - Vce - diode forward drop = voltage across the resistor
5 - 1.4 - 1.3 = 2.3V across the resistor
So the resistor needs to be:-
2.3 / 0.9 = 2.5 Ohms
That is a silly value and points you to the fact that you are doing it wrong. In fact with large currents down an LED you need a constant current supply to drive it.
However, the whole concept of driving the LED so hard is wrong, what are you doing with the LED that needs driving it for so short a duty cycle and as hard as this?