For my money, the best uniform tradition in sports is soccer's custom of putting stars on team crests in honor of past championships. (For a club, that generally means domestic titles; for a nation, World Cups.) It appears that a version of that badge of honor may be coming to NBA jerseys next season.
You can see the gold mark, too, in images of Celtics and Lakers jerseys.
This comes from designer Conrad Burry, who (along with @TheNBALatest) is sharing a whole bunch of stuff from the Adidas Spring 2015 NBA catalog, which apparently went out to merchandisers today.
(Burry already answered your immediately pressing questions. No, ABA titles don't count, so the Nets don't get a gold mark. Some pre-relocation titles count. For example, the Hawks get a gold mark for their title in St. Louis, but the Thunder don't get one for the Sonics' championship, because that would cause rioting.)
I think it's a great idea. A championship really should be cause for lifetime bragging rights, even if it didn't happen in your lifetime. The NBA's drastic lack of parity means being in the title club is a real accomplishment—by my count, 14 teams wouldn't get to wear the gold marks on their collar (15 if the Kings don't count the 1951 Rochester Royals championship).
Hell, take it a step further and start crediting multiple titles somehow. The Celtics and Lakers don't deserve to be lumped in with the likes of the Bucks.
There is, as of yet, no official confirmation from Adidas or the NBA that this is happening, but they'd be dumb not to do it. I can't think of a smaller uniform change that would spur more fans to buy jerseys they already own, just for that tiny, golden validation.
Update: Uni Watch has more.