Thanks so much for all the replies. fastpakr was exactly right! It seems to be an issue which is connected to files not having an ID3v1 info present. This info can be left empty but it has to be applied to each file. I'll post an app to fix this for Mac OS X in my signature!
The software I used is "ID3 Editor 1.9.7" which can be downloaded for free
here. It integrates very well in iTunes (tested with v7.5) and creates miniPlayer valid files with 2 clicks [1.) ID3v1 2.) Update - it's as easy as that!

]!