From a historical standpoint, no accessory has received the pre-order treatment. They have always been available first-come first-served. I doubt this will change with the iPad accessories.
See? I would have gone the other way on this. Apple has made their accessories a "big deal" through the keynote and the preview video (though, no, not in the ad we've seen). Based on that, I'd expect Apple to allow pre-order of their accessories. I'm hoping so, anyway. I'd like to order a case and a dock with my iPad tomorrow.
If Apple doesn't allow pre-ordering (not reserving, but pre-ordering) of accessories it will create an ironic situation -- people who wanted to avoid going to the store will have to go to the store. And based on the iPhone, the accessories are always in shorter supply than the device itself. Not good. So hopefully, if no pre-ordering tomorrow then at least sometime before launch.