I just installed Gravity Forms (very impressed by the way!) and everything worked fine (which is the weird part).
I started creating a form, everything worked, until I added conditional logic, then the preview quit working altogether.
I assumed this was a plugin problem so I started testing plugins, anytime Sexy Bookmarks is active the form preview failed to load, even if it and gravity form were the only two plugins I had.
So I started testing some more since it was working earlier, and it turns out if I remove all condition logic everything works fine again...