from the link you posted:
which further proves my point. the big companies will only bother to notice you when they “start feeling the effect” - i.e. you pose an actual threat to their enterprise. game developer hobbyists should continue on without worry of such things, as there is little to 0 chance of their game being sued in today’s game market.
if you’re actually experienced enough to make money off of your game and pose a threat to the big companies, then you should be smart enough not to use stolen artwork for your professional attention-gathering game.
edit: now that I think of it. BeetleMania, like Geometry Wars, is just a mini game inside a large commercial game. I would receive a cease-and-desist letter too, if they were still selling copies of the game. Grid Wars is a victim of bad timing, really. It should also be noted that Mark Incitti, creator of Grid Wars, was not sued.
so, worst case scenario. if you somehow posed a big enough threat to a big company, they will ask you to stop. if you comply, no harm no foul, and you walk away with the experience you wanted to gain in the first place. of course, SOME companies (perhaps backed by the RIAA ;)) would sue IMMEDIATELY, but again, you’d still have to be good enough to attract the attention.