For what it's worth, I've never actually played Natural Selection. Half life under linux is a pain in the ass. Actually, forget the linux part.
Honestly I find these threads funny more than anything else, but I do have trouble understanding the mentality behind some of the posts I've read on various forums. Alien vs Human games are nothing new; AvP, Starcraft, Gloom, HL, Doom, Q2, Unreal etc. What is relatively new is the FPS/RTS genre of games. Technologically speaking this represents nothing groundbreaking -- it is the natural progression of gaming genres. Where NS differs is the resource system and commander mode features which have their roots firmly in RTS territory. Tremulous is quite dissimilar to NS here, but even if it weren't why would that be a problem? At the end of the day the result for the "mod consumer" is a wider variety of games to choose from -- apparently this is a bad thing. You can go round and round in circles for weeks identifying the origins of entity X from entity Y influenced by entity Z. The reality is that nothing in this world is truly original and if it were we wouldn't be making very much progress. Having said that Tremulous has nothing to do with NS, any similarities are coincidental. As already pointed out in this thread, the inspiration for Tremulous lies with Gloom more than any other game/mod and I'm happy to admit that. If NS was an influence I would say so too, but it quite simply isn't.
The people talking about legal action really need to be dragged down to earth. For a start, computer game modifications are not for profit -- there is nothing to be gained or lost financially before even considering legal costs. Secondly, you can't copyright an idea, only names, artwork, sounds etc. -- i.e. the actual tangible entities that make a product. Imagine if a car manufacturer claimed that legally no one else could make cars: there would be zero consumer choice and under developed automobiles to boot. Screaming "OMG OMG SUE TEHM THEY COPIUED NS!!11!!" only really serves to identify how utterly retarded you are.