Place flank steaks in a large zip-top plastic bag.
Combine beer, Italian dressing, and fajita seasoning; pour evenly over steak. Seal bags, and chill 8 hours, turning occasionally.
Remove steak from marinade, discarding marinade.
Prepare a hot fire by piling charcoal on one side of grill, leaving other side empty. (For gas grills, only light one side.) Place food grate on grill. Place steak on lit side of grill. Grill, covered with grill lid, over medium-high heat (350° to 400°) about 20 minutes or to desired degree of doneness. Let stand 10 minutes before slicing.
Note: For testing purposes only, we used Bolner's Fiesta Brand Extra Fancy Fajita Seasoning, which is available at Wal-Mart and most supermarkets.