trademarks are granted per industry.. so although Nissan Computers doesn't infringe and gets to keep the name... if he changes his site to a car electronics sit

trademarks are granted per industry.. so although Nissan Computers doesn't infringe and gets to keep the name... if he changes his site to a car electronics site (as you suggest), that may allow Nissan Auto to take him back to court and take the name. After all, that would be Nissan Computer Corp changing industries to benefit from Nissan Auto's trademark... and that change would have definitely occurred after Nissan Auto was in the market (meaning he has no right to "Nissan" in the auto industry).