I'd like to be able to sort and store several copies of a song in different file formats. For example, I might want to buy a Coldplay song off iTunes. The 256kbit/s AAC version sounds great, but it's almost 10MB, so I want that on my PC. I might want a converted 128kbit/s MP3 version for my device to save space. I want both in my Amarok collection, but want them to be grouped together, and perhaps have Amarok automatically determine which version to play or to transfer.

Seconded. This would be a verrrry useful feature.

I was actually just thinking of how useful this would be, as I usually store FLACs on my external harddrive and mpcs on my laptop.