For some reason Unity has started reimporting all of the hierarchy assets on a seemingly random basis whenever I go back to the editor after editing scripts, etc. It isn't doing it every time but every few times. I can't quite nail down a reason why.
Any suggestions on what to look for that could be causing this? I don't have any scripts in my Editor folder that would be doing it and I haven't touched those in weeks anyway.
Something might be affecting the library files, might be a virus scanner or if you're sharing your project folder with someone. Unity will reimport assets if there is a mismatch with the library.
I started having this problem this morning, after working with Unity 3 flawlessly since launch.
It may have been because I was exploring the "Preferences" menu last night, as that was the only unusual non-process use I'd made of the interface in the whole build cycle (now nearly a month).
My solution was: export the project entirely, uninstall Unity from my machine, restart my PC, and re-install Unity.
It is now working again.
I'm using Windows 7, 4gb RAM, Boot-camped iMac, running off a default Unity 3 Free.