I would make a daily twitter comment about something cool that happened in a play test.
I would double the articles on the wotc site... every week day something new about the new edition.
I would put up biweekly pod casts talking and joking about the game and the big threads on enworld and there own site.
I would run a short "Happy Birthday D&D" with a wrapped box that says don't open till Gen con, saying a new edition is coming, and put it in comics, and online.
I would put a big banner up on the wotc site "Always, and still the best selling RPG on the market."