Update: Appears to be fixed as of iTunes 10.6 (possibly earlier).
Lets say you want to try out some music on iTunes. Thanks to recent licensing by Apple, you can now preview to up to 90 seconds of a song in the store before you buy. So, you pull up one of the albums and click the "Preview All" link (another recent, welcome feature) to preview all of the tracks. Why, with a 10 track album, you can now listen for 15 minutes for free! So, with that playing, you hide the iTunes window and switch back to your work.
Several minutes in (lets say 10), one of the tracks catches your attention and you switch back to iTunes to see which track is playing. But when you do, iTunes reloads the page you were looking at, stopping your music preview. So now, you don't know which track had been playing or where to start playing from again.
It seems that iTunes wants to forcibly reload whatever store view it is showing after 10 minutes. So, if you've started previewing music and 9 minutes later switch back to iTunes, it won't reload; but if you switch back to it 10 minutes and 1 second later, it will. Leaving the hidden iTunes app completely alone (by either not switching back to it) for longer than 10 minutes will not interrupt your playing preview.
I've noticed this behavior only happens when the app is hidden for more than 10 minutes. If you have the window visible and just switch away from it for 10 minutes and switch back to it, it doesn't reload. It also happens if you "shrink" the iTunes window to the mini-player form and restore iTunes after 10 minutes. Interestingly enough, it doesn't happen when you minimize iTunes and restore it after 10 minutes.
I think one simple fix here is that iTunes should not reload when music is being previewed. Or, simply don't reload any view of the music store, period! My guess is this reloading function was added mostly to benefit the "Ping" feature of iTunes which by nature has more dynamic content and could benefit from semi-automatic reloads like this. But there's a problem there too: if you started writing a comment to post, but then hide iTunes without posting it and come back 10+ minutes later — POOF! — the comment you wrote is gone.
My thanks to Walt Grayson who brought this annoyance to our attention.
(Observed in iTunes 10.2.2.)