See this article: iPad: Using the iPad Camera Connection Kit
No, this canot be done, afaik. When your camera or card is connected, you will see the thumbnails in the camera pane, but not full size preview. So to browse your photos on the iPad you will need to import them to your camera roll. But you need not delete them from the camera or card after importing, so you might simply import selected photos to your camera roll from the camera grid and then delete them again from the camera roll after importing and browsing, without removing them from the card/camera.
What Leoni says is the only way to do it (unless there's an app out there that works better than the photo app)
Import, preview, delete.
The iPad photo app will not display anything but thumbnails off the card.