I've been doing flash development since 2001 but if I had to build a 3D game today, I would definitely be taking a close look at building it using Unity.
Flash developers used to get a lot of flack for not being "real" developers but one thing I've always enjoyed about the community is that they tend to stay on the cutting edge of multimedia technologies. A great blog about Unity3D comes from a developer who used to be heavily involved in Flash 3d work: http://everydayflash.com/
Actually, it looks like more of a new platform similar to OpenGL/DirectX than a 3D engine a la Unreal/Source/IdTech which Unity3D is closer to.
It's not a response to Unity3D, it's a feature that all of us Flash devs have been requesting for years as Shockwave/Director had it, and Flash had taken over its space/niche/functionality.
If you noticed, the demo was built with the Alternativa3D Flash 3D engine. You'd still need to combo with Flash 3D engines like Alternativa3D & Papervision3D to make your life bearable when building complex Flash 3D apps/games.
It would be totally awesome if Unity3D would export to Flash 3D/Molehill in the future! :D
I view Unity3D with it's web viewer as more of a platform than a 3D engine.
Here's a link to an art project built with Unity3D (it may take a while to load - I used Firebug's Net tab to make sure elements were still loading) which I think shows Unity3D is much more than just a 3D game engine and definitely a challenge to flash:
http://www.everyday3d.com/works/installation/colors/
I have one client for which I built an interactive 3D visualization system. The project used Papervision and after trying numerous techniques to minimize the load it took on slower computers we ended up having to make it a 2D system because flash couldn't display everything we wanted to show effectively (it's also possible my Papervision skills aren't up to par and a Papervision expert could make the project work, I still have a feeling the visualization would still need to be limited). Molehill would probably really help this project out.
After building the above system, I started seeing more and more information about Unity3D. If I was to start the project again today, I'm pretty sure I would look heavily into using Unity3D over Flash for the visualization system.
I've been doing flash development since 2001 but if I had to build a 3D game today, I would definitely be taking a close look at building it using Unity.
Flash developers used to get a lot of flack for not being "real" developers but one thing I've always enjoyed about the community is that they tend to stay on the cutting edge of multimedia technologies. A great blog about Unity3D comes from a developer who used to be heavily involved in Flash 3d work: http://everydayflash.com/