Winamp, one of the greatest mp3 players of all time, popularised the concept of GUI Skins. Lo behold we had gazillions of skins floating around the internet(s). It was nice in the beginning but the reason I used Winamp for a very long time was because it was small, simple to use and did the job perfectly. However the skins were a feature that I did not miss in winamp. Why? Simple the default interface itself was pretty good and extremely intuitive.
Lately though, skins are considered a must. I get the feeling that the developers implement it because they can then ignore designing a good UI completely. While this is a clever idea, it is a poor substitute to shipping a complete product. End users understand the product by the UI. So it HAS to be polished. Infact have standard UI and THEN provide skinning. If I have to help someone with finding out where so-and-so button is located, skinned GUI makes things worse. So provide a standard way of switching to the default UI.
On the other hand, if you want to go down the path of skins consider it beta testing for the UI. Simply provide a community website where people can upload their skins. Rank them according to their popularity and then make it the default skin.