The principal value of arctan(infinity) is pi/2. Arctan is defined as the inverse tangent function on the range (-pi/2, pi/2). This means that x = arctan(y) is the solution to the equation y = tan(x), where x is defined as being between -pi/2 and pi/2.
As y = tan(x) is a periodic function, there are infinitely many values of x that would satisfy the equation tan(x) = infinity, including x = -3pi/2, pi/2, 5pi/2, 9pi/2 and so on. To avoid confusion, most mathematicians define the range of arctan as (-pi/2, pi/2), which means that the answer to x = arctan(y) is always a value between -pi/2 and pi/2.