Like CaptainFerret said whilst thigns appear to work depending no the skin you're likely to get some serious problems here and there.
Skins work as a hierarchy. It looks in you skin folder and if the file isn't there is uses the default file from the data folder.
Lightweight skins with a few changes might appear to work but anything with match screen changes, changes to files in the settings folder (text colours etc) or indeed minor chages to a few screens means it's likely to break or you'll lose new features.
Same as it was for FM07. There was a glut of 'conversions' of older skins and none of them were done right. They all had daft issues - buttons missing form screens, wrong text colours.
"
I MADE ALL 2007 SKINS WORK ON 2008!!!!!!!!" is stretching the truth.