Credit: Tesla.Yahoo
My guess: "D" is for dual, as in the planned all wheel drive option for the Model S, using dual electric motors, previously mentioned on these pages.
http://www.businessweek.com/articles/2014-10-02/tesla-teases-its-next-big-d
http://www.greencarreports.com/news/1094732_elon-musk-tweets-tesla-to-unveil-d-on-oct-9th-our-guesses
https://autos.yahoo.com/news/tesla-showing-everyone-d-october-9th-133052927.html
Update 03 October:
Credit: adelman/Tesla Motors Club/Engadget
Zoom in on the badge on the right. "P85D" implies an 85 kWh battery, & so, that speculation about "D" being related to increased battery capacity, is wrong.
http://www.engadget.com/2014/10/02/tesla-model-s-p85d-leak/ / Tesla Motors CEO Elon Musk Twitter tweet tweeted photo caption About time to unveil the D and something else roller letter D shadowy front end Tesla Model S date October 9 Tesla logo retweed retweeted favor favour favorited comments announcement updates new options EV electric luxury sedan vehicle 4WD four wheel drive all-wheel drive option Model X crossover utility vehicle new electronic safety systems adaptive cruise control lane departure warning additional sensor hardware retrofitted existing cars via over the air software updates larger battery pack option rumored 110 kiloWatt hour pack production version /
