I haven't seen the AI nuke anyone in regular BTS, but I'm playing the Next War scenario, and I've been nuked a couple of times. Just tactical - probably because ICBM's aren't all that great, because SDI can stop half of them, and with all the shields and stuff you can build in that mod, nukes aren't too great at mass destruction of cities. But the Lady (Yellow, controls South America, Middle East and Africa in this mod) did use tacticals against me twice, once ineffectively against one of my cities, the other very effectively against my stack that had just taken one of her cities. (The stack survived, even after she launched a counterattack in conjunction with the nuke, but she managed to kill 3 out of 4 of my Assault Mech's, including one with a Great General attached Set my assault back 10-15 turns. I've seen fallout in other places on the globe, so I'm pretty sure the other AI are nuking each other as well, even if it's just with tacticals. Again, this isn't the regular epic game, though.