Heath Ledger as The Joker.

My impression?
Mmmmmm.... geeky goodness.
I love the cut smile, and the insane look of it.
I can't wait to see more.
Here's hoping that this Joker will be less of a funnyman, and more of a homocidal psychotic sociopath.
*fingers crossed*
A couple of links for those of you descending into Batman sequal madness, as I am:
Batman-on-film (news and scoops)
The Dark Knight (Official site)
I Believe In Harvey Dent (Gotham City District Attorney "campaign" page)
Up until this weekend, the film was using a little viral marketing to tease the big reveal of the Joker image you saw above.
Several businesses found themselves littered with playing cards that looked like this:

They have "I believe in Harvey Dent Too" written on them.
Until today, you could visit the website www.ibelieveinharveydenttoo.com and see a version of the "campaign" site that looked like it had been hacked and defaced by the Joker. At the bottom of the page you could enter your e-mail address, and it would send you an e-mail with X and Y coordinates in it, and a link back to the page. When you entered the coordinates on the page, it identified that ONE PIXEL, and revealed what was behind it. Thousands of e-mails later, the picture underneath was revealed to be our first look at Heath Ledger as the Joker.
Genius.
**EDITED TO ADD:**
I went back to www.ibelieveinharveydenttoo.com a minute ago.
Even though the page content appears to be removed, and the page says in red letters "Page Not Found" - if highlight over the entire page, it reads "HahaHahAHAHaha" all over it. VERY disturbing.