Dan,
EDIT: Never mind, I just saw your other post re: bug fix slated for release today (May 2). Please disregard this post; I'll just wait patiently I just bought this on Steam (which is where I discovered it!) after downloading the timed demo from your site. I have to say, it's really very cool!
However, when I download it on Steam, I'm told it will require 350+M of space, but Steam only downloads 150M or so. Then, when I start the simulation, it hangs on the loading screen with the rotating sun and orbiting planet icon- and there it sits. Once, the hard drive ran and ran for a bit, but nothing else happened.
I tried deleting the local content from within Steam and reinstalling, but that didn't seem to do any good. I'm running Windows 7 Ultimate x64, have the latest drivers and Windows updates, etc. I'm confused because the timed demo ran fine. Should I be doing something else that I'm not aware of- maybe delete the local content, THEN reboot, THEN reinstall?