The iPad will never be over taken. There may be competitors, but no one will compete with a me-too device. You cannot take on originality with a copycat. The only advantage would be a few features and cheaper price. Not to stifle competition, but until someone can develop a better OS, all the hardware will do little good.
Its an interesting and creative idea. It wont sell 3 million regardless of how good or not it is because it doesn't have an apple logo on the back. Despite what you may have read, windows 7 can have a quick startup time and decent battery life, particularly when used with solid state drives instead of conventional hard drives (like the iPad). Its not instant-on, but I suppose you could write your OS to ROM and get that as well.
The bottom line is that we will just have to see when it comes out. I dont know about "knocking the iPad off its pedastal". Its a different kind of device and will probably hit a niche market that the iPad doesnt. It will have real windows, can run real MS Office apps, etc. Its more like a snazzy netbook than an internet appliance like th iPad. I forsee this slate form factor coming out in all kinds of flavors and specialized uses. I dont really care how any of them sell. I wont be shopping for anything like this for a while - I have new iPad.
The Toshiba Libretto is an interesting gadget but it performs a very different function from the iPad. The Libretto is after all still a netbook. Win 7 is not designed specifically for the touchscreen as opposed to iOS. The software developers will not specially develop touchscreen Win 7 software for the Libretto as well. Having a dual LCD screen will also mean that the battery life will suck (I believe battery life for the Libretto is about 3 hours). I doubt if the Libretto will be cheap, so it will remain a niche gadget. It will not pose to be a threat to the iPad for sure.