I was actually doing some research on the same thing. I found something called the "Amazing Slow Downer" from Roni Music.
http://www.ronimusic.com/
However, on for the iPhone / iPod Touch version of the software, I found the following disclaimer:
Important notes:
1. Requires a Wi-Fi network connection - Please read more here about uploading files to your iPhone
2. It's not possible for third party applications to access songs already loaded into the iPod application. You must import songs from your computer into the Amazing Slow Downer application via a Wi-Fi network connection.
3. Cannot play DRM protected files from the iTunes store (m4p files and audio book files).
I don't have an iPod Touch yet, and I was contemplating buying one so that I could use it in conjunction with this type of software. But from the disclaimer, it seems that you cannot use the software directly on your iTunes files. Rather, it seems that you have to load the files through the software onto your iPod. (If anyone with experience knows if I'm wrong, please correct me.) Essentially, I want something user-friendly that's not going to be a pain in the rear end to use.
Also, don't have wi-fi at home. Don't know if there is a workaround (ie, downloading the application and transferring files via a computer into the iPod).