I found a site that has 13 clips from The Dark Knight that run about 30 seconds to a minute each. All of the clips are high quality, which leads me to believe that they're officially sanctioned previews and won't be immediately yanked by Warner Bros, but you might want to check them out soon just in case. There isn't anything in them that's a major spoiler, but they do add up to about 10 minutes of footage so if you want to go into the flick completely cold, I'd avoid 'em.
Some things we learn from these clips (minor spoilers):
1. Anyone who didn't like Christian Bale's scary voice from Begins (I didn't mind it so much) will be disappointed. It returns.
2. If the brief fight scene in one of these clips is any indication, Nolan's greatly improved his fight direction. It also helps that Batman's new costume is a helluva lot more flexible than the last one.
3. Not that anyone needs more convincing, but Ledger's Joker really is unlike any other performance of the character. Darker, quieter, a little more grounded, and much scarier.
4. Apparently one of the Joker's big schemes is that Batman needs to go public and reveal his identity or he promises that (according to the very first teaser) "starting tonight, people will die".
5. Nolan must shoot a lot of takes, because many of the most memorable lines from earlier previews ("The dawn is coming." "This city deserves a better class of criminal. And I'm gonna give it to them." "I'll settle for his loved ones.") sound completely different in their final version.
6. Taking place just after the end of Begins, neither Wayne Manor nor the Batcave seem to be rebuilt yet, so Batman has a new lair (we've seen plenty of it in other previews already). He has, however, started putting together his classic, huge, multi-screen Bat-computer.
By the way, scroll down through the site if you get the chance. Not only has this guy collected every trailer and poster that has been released (and some that haven't), but he also has a clip on there that I haven't seen anywhere else - one that shows the briefest of glimpses of Two Face himself.