In a small bowl, combine chili powder, cumin and cayenne. Drizzle steak with lime juice. Rub spice mixture into steak and place in a large resealable plastic bag. Seal and refrigerate, turning occasionally, for at least 2 hours or for up to 8 hours.
Preheat lightly greased grill to 400°F (medium-high setting).
Grill steak, with grill lid closed, for 5 minutes on each side or until desired degree of doneness (5 to 6 minutes per side for medium, 6 to 7 minutes per side for medium-well, 7 to 8 minutes per side for well-done). Remove steak from grill. Let stand for 5 minutes, resting in juices. Cut steak diagonally across the grain into thin slices. Serve in tortillas with lettuce, tomatoes, cheese and desired toppings.
TIP: Alicia's Guacamole recipe goes great with this one!