I have a little information to add on this. From what I've gathered on the forums and in the FAQ, the belief seems to be that an iTunes (for Windows) bug causes the resets.

However, I'm running Mac OS X 10.5.6 (and iTunes 8.0.2, if that helps anything), and I had what sounds like exactly the same experience: I was playing along happily, then suddenly, the game reset completely, as though I'd pressed the Start Over button. Fish, tanks, ornaments - everything was gone, just as others have described here.

As it happens, I'd been showing a friend the game a few minutes before, and it had been fine. I saw the App Store had updates for a few of my apps - I allowed it to run, and went to check back on my fish, which is when I discovered the game had reset. So it seems the bug is not isolated to iTunes under Windows. I've never changed the time setting on the phone's internal clock, just to throw that out there.

Mine's an iPhone 3G running 2.2.1 - latest available as of this posting, and the version has not changed since I installed Fish Tycoon. I've synched many (MANY) times with my machine since I installed the game - the problem only occurred immediately after I ran updates from the App Store application on the phone itself.

Hope this helps the developers - and of course, I hope there's a patch soon. I love the game, but it sure is disappointing to lose days' worth of work.