Simulating mouse input programmatically in OS X

Yes, it is possible. You can use the Quartz Event Services to simulate input events.

Assuming C, I wrote this quick example:

#include <ApplicationServices/ApplicationServices.h>
#include <unistd.h>

int main() {
    // Move to 200x200
    CGEventRef move1 = CGEventCreateMouseEvent(
        NULL, kCGEventMouseMoved,
        CGPointMake(200, 200),
        kCGMouseButtonLeft // ignored
    );
    // Move to 250x250
    CGEventRef move2 = CGEventCreateMouseEvent(
        NULL, kCGEventMouseMoved,
        CGPointMake(250, 250),
        kCGMouseButtonLeft // ignored
    );
    // Left button down at 250x250
    CGEventRef click1_down = CGEventCreateMouseEvent(
        NULL, kCGEventLeftMouseDown,
        CGPointMake(250, 250),
        kCGMouseButtonLeft
    );
    // Left button up at 250x250
    CGEventRef click1_up = CGEventCreateMouseEvent(
        NULL, kCGEventLeftMouseUp,
        CGPointMake(250, 250),
        kCGMouseButtonLeft
    );
    // Now, execute these events with an interval to make them noticeable
    CGEventPost(kCGHIDEventTap, move1);
    sleep(1);
    CGEventPost(kCGHIDEventTap, move2);
    sleep(1);
    CGEventPost(kCGHIDEventTap, click1_down);
    CGEventPost(kCGHIDEventTap, click1_up);
    // Release the events
    CFRelease(click1_up);
    CFRelease(click1_down);
    CFRelease(move2);
    CFRelease(move1);
        return 0;
}

And assuming GCC, compile with:

gcc -o program program.c -Wall -framework ApplicationServices

Enjoy the magic.