I discovered this yesterday, as well. It wasn't working through allTunes, but I was able to get preview working on the mp3sparks site.
Today, however, neither is working.
Given that mp3sparks is about the only full-preview store out there, I wonder what it is they felt they needed to improve?
Well, I'm subscribing to this thread and hopefully somebody will post the good news soon that the previews are working again. I can't imagine they would think it's good to have them entirely down for long -- I suspect their sales will slump at least a little bit if prospective buyers can listen to the material first. (I always listen to every track in its entirety several times, and ultimately buy only the ones I like, or at least don't dislike -- but I may be in the minority here.)