There is one more weakness that might possibly arise on Apple's side, and that is selection. Apple is rumored to be scrambling for content deals, while Amazon is trying to make sure that their Kindle program sits front and center everywhere it can. If Apple can't lock down content the way it wants to, Amazon and Barnes and Noble may still have a chance to secure an audience. That, combined with their lower price and more focused nature as an e-reader, could allow them to portion off some of the market.
It's going to be a tough road ahead for anyone trying to compete directly with the iPad, though. Pre-orders have already been big, and as the devices make their way into the real world, sales will likely get even bigger.