Here is the official logo and some concept art, including the main character's ship, called Ghost. The art is very McQuarrie-esque.
Here is the Star Wars Blog's coverage. Evidently the show will take place closer to ANH than ROTS, but it sounds from various sources that Dave seemed coy about whether or not he'd bringing in Clone Wars characters, so it's seeming a little likely.
Bleeding Cool has more updates, including hi-res photos of the concept art shown at the panel.
There's also an exclusive Rebels logo T-shirt that sold out in person within minutes but is available through their site.
Here is some more artwork and video a fan took at Star Wars Celebration Europe: