It's great that more "usable" portable computers are coming out. MBA was the first balanced portable computer, Windows or Mac, in my opinion.
Obviously this was not a copy cat attempt by Lenovo, as it would have taken longer than 2 months to put a computer out in the market. Regardless, competition is good for us and for the industry as whole. Expect better and lighter machines in the future. Not smaller, I don't think, because there is a limit on how small it is without sacrificing usability. UMPC are not as popular because of this. I had the Sony UX series and it was awful to use and to see the screen.
I would buy the x300 if it was better than the MBA, but it really isn't... Not for my use. Maybe for corporate use. I prefer OS X, but if the hardware is good enough, I wouldn't mind getting a Windows platform in the future.
I just have to keep making more money so I can keep up with the new stuff!