I've just tested the latest Wubi.exe available here. It took a while to download the latest daily-live Desktop CD image (in the end I did that myself from here). But after that installing Wubi was painless. Apart from a benign grub2 message, the Ubiquity installer was impressive - looking ready for release - and seemed faster than prior releases, as did booting up for the first time.
Surprisingly Unity even worked on my computer - I'm not totally sold on it and there were some rendering issues, but at least now I have the opportunity to review it, instead of staring at a blank screen.
So - in summary - the installation of Wubi Natty Narwhal worked flawlessly. As always, alpha testing has risks, so take necessary precautions.