Instead of reducing the number of plug-in formats, Mac OS X has added one more to the mix. Furthermore, many Windows-based developers have not found the AU format easy to develop for, and the rollout of AU plug-ins has been slow. Finally, as with other plug-in formats, not all AU plug-ins function equally well in all host applications. Read the article here. Also, make sure you read MacDevCenter, as it contains a number of interesting Mac development articles lately.