how about this one:

"Sail on for the dawn!"

or:

"Dawn of the voyage!"

serious:

"Everything for the Utter East"
"The Sun Gets Brighter as the Journey Begins"

for a poster of Eustace:

"You've always wanted to visit Narnia. He hasn't."

edit:

The tagline for LWW was "Defeat evil, free Narnia!" and PC's tagline was "A New Age Has Begun". I suspect VDT's tagline will be similar; not too original but not cringe-worthy.
And I actually don't care. The Chronicles of Narnia: The Voyage of the Dawn Treader is enough of a mouthful without a tagline and pretty much explains itself.

lol, good topic idea!

Serious:

The Voyage Begins.
Set Sail On Dec. 10th, 2010. (A little lengthy...)

Bad and Cheesy:

Hope for sailing, not swimming.
Bring a life jacket.
Take a breath, and hold your nose!
Plunge with the Dawn Treader! (Uh, no!)
The Utter Eastward End of the Series!
No Vampires Here! (lol)
Be Prepared To Get Waterlogged!
Bye Bye Disney. (lol)

lol, those are really cheesy and just flat out terrible.
I wonder what the real tagline will be... I can't wait to find out!

Thanks for posting the taglines for LWW and PC, Lucy P., until today I didn't even know what a tagline was. (Yeah, I'm that clueless ).
I wouldn't be surprised if they worked Voyage into it some how or something about "setting sail" or "Sailing".
Here are some ideas (I don't know if they are good or not (I'm still not 100% clear on taglines)).
"Set sail to find all you seek" or maybe just "Find all you seek"
Seek the utter East. (they could even add "with Reepicheep" to get in more of the "e" sound )

Lucy P. I love your idea for the poster of Eustace.

funny:

"Sailing into the sea (quite literaly)"

"Prepare for a wetting!"

"Danger: you might fall into the ocean through a picture frame"

How about "a voyage of unexpected discoveries"?

As long as the tagline doesn't include the word "unfathomable," I think I'll be okay with it.

Taglines are tricky. It's really hard to do them without being totally dumb. But in the rare cases where they work well, they're very cool. The "Watchmen" posters were one of those rare cases.

"Everything you know is about to change" for PC was really dumb.

As long as the tagline doesn't include the word "unfathomable," I think I'll be okay with it.

Taglines are tricky. It's really hard to do them without being totally dumb. But in the rare cases where they work well, they're very cool. The "Watchmen" posters were one of those rare cases.

"Everything you know is about to change" for PC was really dumb.

Yes it was. Aslan said it in the trailer, Cornelius said it in the film, and early posters had it slightly different - "Everything you know is about to change forever." Too melodramatic, inconsistent, and disconnected from the story.

My preference would be something simple: "The Voyage Begins December 2010."

how about this one:

"Hitting the sea head on December 2010"

For me, a good tagline should try to convey to the viewer some of the main themes and ideas behind the movie that they would perhaps not get from the title/poster alone. Thus for Prince Caspian, the taglines were all about trying to convey the idea that the movie was alot more grown-up and mature than the first film. Effectively they were trying to sell the film to the large number of people who went to see the first film expecting something on a par with LOTR but came away disappointed that it was too bright, colourful and childish, and would perhaps not be willing to give a sequel a second chance. Thus thus taglines contained phrases such as "...A More Savage Place...." and "...Will Change Forever"

With VDT i think it will be the exact opposite, and they will be targeting the marketing at the people who liked LWW, but perhaps felt alienated by the darker adult tone of Prince Caspian, so i would expect the tagline to convey something along those lines - that not only is the magic "back", but that everything is even more magical and wonderful and fantastical than it was in LWW. Hence i quite like coracle's suggestion:

How about "a voyage of unexpected discoveries"?

or perhaps something like "Journey Beyond Your Imagination" or similar that sells the idea of a return to the sort of magic and wonder seen in LWW. It probably doesn't need to be the greatest tagline in the world ever, as long as it does the job of selling the film in the right way it will be fine, but of course it wouldn't hurt to have one that did it in a short, smart and memorable way.

The Internet Movie Poster Awards website has annual award categories for best serious tagline and best funny tagline. They are often rather quite clever: http://impawards.com/awards.html

how about this one:

"All hands on deck December 10, 2010"

How about one-word character posters?

Sail. (Caspian at helm, wind blowing)
Search. (Reepicheep in his coracle, from a distance)
Spar. (cool one of Ed ready to fight)
Soar. (Lucy watching the white bird fly away after Dark Island)
Transfigure. (close-up of Eustace, half of his face dragon and half human)

I like those one word ones, Lucy P. Nice!

Lucy P.: awesome! that's a great idea! I like the ones you came up with!