Mac DevCenter has a really cool tutorial up on their site. The how-to outlines the steps your can take to actually download the 30 second song preview in iTunes. These 30 second previews can then be put onto your iPod, so you can preview them there—maybe on-the-go—before you actually decide to go and buy the full track/album. This process uses the command line in OS X to to download the files from their URLs. Check it out before Apple decides to pull hit the tutorial with a fake cease and desist.
??? I'm almost certain that the time it takes to download it from iTMS, and then up load it into your iPod, is more than 30 seconds, so, why would anyone want to do this?
Well exnihilo, if you read the tutorial, it begins with a few answers to your question:
I see, well...
1. This seems to be a Rube Goldbergian way of doing this. Just use Audio Hijack. The resulting AIFF file for a 30 second tune isn't that big, so you won't have to recompress it to AAC.
2. The article says the track may sound different on the iPod. That's mainly a function of the headphone. Just listen to it with the headphone you'll be using.
3. I'm a geek who values his time. Audio Hijack seems a lot less time consuming that the method described.
But hey, whatever floats your boat.