Flex 4 has introduced an entirely new and improved approach for creating custom components and skins with the Spark component model. The main take away is that behavior and presentation have been entirely decoupled.
I am going to keep a running list of my favorite articles about said topic as I have set out to wrap my mind around this new and exciting paradigm.
Dan Orlando has written a couple fantastic articles on the subject
Building Custom Components with Flex 4
Christophe Coenraets also written a super article
Creating a Custom Component and Skins in Flex 4
And Ryan Frishberg gets a high five for his article
Introducing skinning in Flex 4 beta (updated for beta 2)
