Someone asked about the curbside buses (actually courtesy vans) and they still run.

I know a few smokers who would prefer to go landside at A, have a smoke, hop the van to D, and reclear security. Security lines at D have never been more than five minutes for me (and usually there is NO ONE in front of me at all) - so going airside and transiting that way shouldn't take more than twenty minutes gate-to-gate. (Or the other "smokers option" would be to use the smoking lounge inside the A Admirals Club.)