The exact same thing JUST happened to me two hours ago. The exact same thing. Bizarre.
I have a 15" MacBook Pro, Intel Duo Core.
I didn't even shut my computer down or restart it...all of a sudden my applications were asking me to configure them, as if I had just turned the computer on for the first time.
ALL of my addresses, mail messages, calendar items are gone...nowhere to be found.
iTunes and iPhoto have been reset too, but luckily the files are still there for me to create a new library from.
When I first noticed that Mail had been reset, I went to restart the computer...but an Error box appeared, stating that the "Mirror Agent" program was in use and would need to be force quit.
Mirror Agent? I don't use .Mac or sync up my laptop with anything...and I've never used this program ever.
Is this caused by a virus perhaps?
Luckily all of my files are backed up, although I failed to backup my Library...so the address book, calendar items, inbox, and stickie notes are gone for good.
Anyone have some advice?
Cheers...
It seems as though my ibook has reset itself for no apparent reason. All of my preferences and keychain passwords are gone and I have no idea why. I just closed the top down, put it to sleep, and when i opened it 5 minutes later, none of the settings i had were as they should be. Everything has gone back to the default settings. The files that I have saved such as my music and documents are all there but they arent where they should be.
For example, all of my songs are still here and in my "itunes music folder" but none of them are actually in itunes when i open the program. I tried to drag the folder into my itunes to get the songs to all go back but it just reminded me that my hard drive is full (500 free mbs) and that it would try to upload as many as possible and proceeded to upload the first 100 or so songs (out of 2000+) before it got full.
I restarted my computer and then everything that was reset became more apparent as I logged in. Even the dock and my desktop are different. The items that I had on my desktop aren't there anymore.
I have a g3 ibook, it is 60 months old, has OS 10.4.8, 20gb HD, 256 ram, 600 MHz processor

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic