As I've been thinking about it some more, I think that to come anywhere close to achieving the original goal of increasing the number of players by 20% in the next 18 months (let's forget dealing with how exactly you would measure that goal - does the base of "current players" include everyone who bought a Player's Handbook or is a more real number based on some kind of surverys?).
Anyway, I really think the way to go is not with traditional media advertising but rather with the idea of the "Demo Teams". You'd break your D&D Demo Teams up into ones that target different groups:
1) Young Kids. You could probably target as young as 10 or so, maybe slightly younger, but with a core of around 10-14.
Find a public space (preferably not a game shop) to have the Demos. We actually did this years ago when I was in High School. We got space at the local library in one of their big rooms with sound-proofed doors so we didn't bother the rest of the patrons, posted flyers, and taught three young kids how to play D&D. And, we did that on our own. So, if it were actually organized, you'd probably get a bigger turn-out. You'd invite the parents as well so they could make sure that their kids weren't getting into anything weird. Have some refreshments (pizza would probably be fine, but some healthy fruit snacks as well). You'd get the library's permission (or wheverer you hosted it - local community center, etc.). WotC could run some small ads in the local newspaper, and the Demo Teams could post flyers prior to the event, and also hang out in places where these kids might be (malls, like at the Computer Game Store, or whatever) and let people know that the Demos were coming up.
For the game, instead of just using the Core 3 books, you would use miniatures and a modified set of rules that's really stripped down but runs well with the collectible miniatures. Each person who attends the demo would get a special miniature, only available to people who attend the demo. The kids would totally love that.
2) College Kids. Basically the same thing as above, but obviously you'd demo the game on the college campus. You would advertise the Demos in the college newspaper and on the local college radio station, plus flyers and stuff of course.
For this crowd, you could use the regular rules (or at least show them) but would be better off using a "fewer options" version just for the sake of simplicity.
3) "Genre" Fans. This is basically a catch-all for people out of school who share similar interests of gamers. Screen for interests in things like CCGs, CRPGs, SF/Fantasy TV/Movies/Books, and Comics. You'd promote the events similarly to the way there were promoted to the younger audience, in the local paper and with flyers and signage.
Really, you could target all three groups in similar type places, but I think it would be important to separate out at least the younger players when running the demos.
Things to Avoid:
1) Celebrities. The second you try to tie a celebrity to the game when promoting it to kids is the second that you've just artificially shortened the life cycle of interest in the game. Celebrities come and go - you don't want to tie interest to D&D to the "flavor of the week".
2) Changing the Game. The game should not be changed to try to capitalize on the latest fad. That's not going to help. Instead, it should capitalize on its strengths (social interaction, imagination, etc.). Also, I think the idea of the kid-targeted miniatures thing really would work pretty well (and kudos to the people above who suggested that - I forget who it was).
There are other obvious things to avoid (mostly having to do with properly screening your Demo team members to make sure they represent the brand properly and can interact well wtih kids and adults, don't have criminal records, etc...) but they're all just common sense things anyway.
I think that if Hasbro/WotC could invest in such a program, get quality people to run the Demos, produce some exclusive miniatures only available to people who attend the Demos, and could find the funds to promote the Demos, they could increase the player base. I'm not sure they could do it by 20% in 18 months, but I'm about 99% certain that they'd get more players than they would buy spending the money on a traditional advertising campaign.
). One thing the old D&D cartoon did do well (could have done better, and could have done it a LOT more faithfully, but it did do it) was that it showcased a lot of D&D stuff. I mean they used the Bullywug, Hooked Horror, and Shambling Mound! How great was that!? 
