At current, I have no idea. I'm leaning toward disallowing them for Forgotten Realms games (timeline advance be damned), definintely tossing those suckers in there for Dragonlance if I play it, and hve no idea for anything else I do. In general, the more over-the-top the setting is, the more I'm likely to allow them.
I don't GM, so I'm not in a position to ban anything, but if I were? Heck no. I'd sooner ban elves, dwarves, and halflings. Lizardmen are way more interesting than pointy-eared humans or short humans.
I will use dragonborn if and when I create a setting which has a place for them. I treat all elements of the game, core and otherwise, in this fashion. Races, classes, whatever. I've got no interest in a "core only game" primarily because Wizards of the Coast has to sell "core D&D" to an audience which likes a lot of dumb stuff I'm not interested in using.
I don't have a particular problem with including reptilians if a player wants one, though the designers will need to come up with something pretty amazing to convince me they are necessary to any campaign setting. I would want to ban the wings and the lightning-struck eggs.